WINDERMERE, Fla. ( Stockpickr) -- Corporate insiders sell their own companies' stock for a number of reasons.

They might need the cash for a big personal purchase such as a new house or yacht, or they might need the cash to fund a charity. Sometimes they sell as part of a planned selling program that they have put in place for diversification purposes, which allows them to sell stock in stages instead of selling all at one price.

Other times they sell because they think their stock is overvalued and the risk/reward is no longer attractive. Some even dump their own stock because they have inside knowledge that a competitor is eating their lunch and stealing market share.

But insiders usually buy their own shares for one reason: They think the stock is a bargain and has tremendous upside.

The key word in that last statement is "think." Just because a corporate insider thinks his or her stock is going to trade higher, that doesn't mean it will play out that way. Insiders can have all the conviction in the world that their stock is a buy, but if the market doesn't agree with them, the stock could end up going nowhere. Also, I say "usually" because sometimes insiders are loaned money by the company to buy their own stock. Those loans are often sweetheart deals and shouldn't be viewed as organic insider buying.

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At the end of the day, its large institutional money managers running big mutual funds and hedge funds that drive stock prices, not insiders. That said, many of these savvy stock operators will follow insider buying activity when they agree with the insider that the stock is undervalued and has upside potential. This is why it's so important to always be monitoring insider activity, but it's twice as important to make sure the trend of the stock coincides with the insider buying.

Recently, a number of companies' corporate insiders have bought large amounts of stock. These insiders are finding some value in the market, which warrants a closer look at these stocks. Here's a look at some stocks that insiders have buying, per SEC filings.

Celgene

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One biotech player that insiders are buying is Celgene ( CELG), a global integrated biopharmaceutical firm primarily engaged in the discovery, development and commercialization of therapies designed to treat cancer and immune-inflammatory related diseases. This stock hasn't done much so far in 2011, with shares off by around 4%.

Celgene has a current market cap of $26 billion and an enterprise value of $25 billion. The stock trades at a reasonable valuation, with a trailing price-to-earnings of 25 and a forward price-to-earnings of 13. I say "reasonable" because Celgene's growth estimates are for 23% for this year and 23% for next year. This is a cash-rich company, with $2.43 billion of cash on its balance sheet and $1.25 billion in debt. After you back out the debt, Celgene has $1.18 billion in cash on the books.

A director just bought 5,000 shares, or $290,498 worth of stock, at $58.10 per share. This same director also bought 10,000 shares, or $515,800 worth of stock, in January, at $51.58 per share. This shows a lot of confidence since this insider is paying up with each purchase.

From a technical standpoint, shares of Celgene are currently trading below its 50-day and 200-day moving average, which is bearish. That said, the stock just found some buying support a few days ago at a longer-term support zone near $55 a share.

If you want to buy this stock, you could add it on any weakness and simply stop out of the trade if you see it move below $55 a share. You could also wait until the stock recovers into a healthier trend and is trading back above the 50 and 200-day moving averages.

Another strategy that has worked well over the past few years is to buy Celgene in the low $50s to high $40s and sell in the mid-$60s. That trend could easily continue since Celgene shares seem to channel within those ranges.

Celgene is one of TheStreet Ratings' top-rated biotech stocks.

BlueLinx Holdings

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One under-$5 stock whose insiders have been buying up a ton of shares of is BlueLlinx Holdings ( BXC), which, through its subsidiary, distributes building products in North America. Insiders are clearly finding some big value in this stock since shares are off over 40% so far in 2011, and the stock is trading at new 52-week lows.

BlueLinx has a market cap of $68 million and an enterprise value of $526 million. This is not a profitable company, and it's bleeding cash. BlueLinx's operating cash flow is -$46 million, and its levered free cash flow is -$38 million. This is far from a cash-rich company, with over $461 million in debt on its balance sheet and just $6.15 million in cash.

A beneficial owner just bought 15.6 million shares, or $32.7 million worth of stock, at $2.10 per share.

From a technical standpoint, this stock has been destroyed in the past couple of months, with shares falling from a high of $4.35 in April to their current level of around $1.95 a share. The stock is currently trading below both its 50-day and 200-day moving averages, which is also bearish.

If you want to buy this stock, I would suggest waiting now that it has broken below some major support levels at $2.07 to $2.13 a share. This break is an extremely bearish move and could mean that BXC is setting up to trade all the back toward its next significantly support zone near $1.02 to $1.20 a share -- which is where I would start to look at this name.

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Infinera

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One name in the communications services sector that insiders have been buying is Infinera ( INFN), which provides digital optical networking systems to telecommunications carriers worldwide. Over 80 customers, including national and multi-national telecom carriers, cable television operators and Internet content providers, use its network architecture in their fiber optic backbone networks. Insiders could be sensing some deep value here in this stock since shares are off by over 30% so far in 2011.

Infinera has a market cap of $738 million and an enterprise value of $480 million. This company currently isn't profitable, and it's bleeding cash, with -$48 million in levered free cash flow. On a positive note, Infinera is cash-rich, with $268 million in cash on its balance sheet and zero debt.

The vice president just bought 150,000 shares, or about $1 million worth of stock, at $6.44 to $6.96 per share. The CEO also just bought 50,000 shares, or $326,005 worth of stock, at $6.52 per share.

From a technical standpoint, this stock is trading below its 200-day moving average and above its 50-day moving average, which is neutral trendwise. Shares of INFN have been making higher lows and mostly higher highs during the past two months, which is bullish. Also during that timeframe, we have seen some big volume up days, which could mean that large traders are starting to accumulate the stock at these depressed levels.

If you want to get long INFN, then you could buy this on any weakness and simply stop out if it trades below its 50-day moving average of $6.72 a share. I would add to any long position only if you see INFN trade above some near-term overhead resistance at around $7.45 to $7.60 a share, and then add again if it gets back above its 200-day moving average of $8.13 a share.

It's worth mention that INFN has a decent short interest -- I highlighted it recently in " 5 Heavily Shorted Stocks Ready to Soar on Earnings." The current short interest as a percentage of the float is 8.4%, and the short-sellers have been increasing their bets from the last reporting period by around 8.3%, or 609,000 shares. A move above those near-term overhead resistance levels on solid volume could spark a short covering rally, so keep this name on your radar.

Opko Health

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Another biotech stock whose insiders are doing some big buying in is Opko Health ( OPK), which is focused on developing a range of solutions, including molecular diagnostics tests, pharmaceuticals and vaccines. Opko operates in two segments: pharmaceutical and instrumentation segments. Insiders are paying up here to own shares of this name since the stock is up over 10% so far in 2011.

Opko Health has a market cap of $1.2 billion and an enterprise value of $1.1 billion. This company is currently not profitable yet. Its operating cash flow is -$21.1 million, and its levered free cash flow is -$22 million. Opko is cash-rich, however, with $107.86 million in cash on its books and just $14.55 million in debt. After you back out the debt, that gives the company $93.31 million in cash.

The CEO, chairman of the board and director just bought 100,000 shares, or $412,064 worth of stock, at $4.12 per share. This same CEO has been steadily buying millions worth of OPK stock since 2010. It's hard to make a bearish case against a stock where the CEO has been buying up shares as consistently as at OPK.

From a technical standpoint, this stock is trading above both its 50-day and 200-day moving averages, which is bullish. The stock has also been making higher lows and higher highs since the middle of June, which shows that the trend is positive.

If you're looking to get long OPK, I would wait until the stock breaks out above some near-term overhead resistance at around $4.40 to $4.50 a share. I would then add aggressively to any long position if you see OPK trigger a major breakout above $5.03 a share on solid volume. Look for a volume move that registers close to or over the three-month average action of 988,000 shares. Another way to play this name would be to buy it on any weakness as long as it holds above its 50-day ($3.83) and 200-day ($3.72) moving averages.

It's worth mentioning that OPK has a decent amount of short-sellers involved in the stock. The current short interest as a percentage of the float for OPK stands at 8%. If you see this stock break out, then expect a sharp short-covering rally to ensue as the bears peel off some of their bets.

RealD

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One more stock that has seen some decent insider buying is RealD ( RLD), a global licensor of three-dimensional technologies. Its property portfolio enables a 3D viewing experience in the theater, the home and elsewhere, and the company licenses its RealD Cinema Systems to motion picture exhibitors that show 3D motion pictures and alternative 3D content.

This is another situation in which insiders are finding some deep value since this stock is down by over 40% so far this year.

RealD has a market cap of $776 million and an enterprise value of $743 million. The stock trades at a reasonable valuation of 20 times forward earnings when you consider that its growth estimates are for 123% this year, and 248% next year. The company isn't cash-rich, with $18.89 million in cash on its books and $26.72 million in total debt.

A director just bought 9,500 shares, or $132,125 worth of stock, at $13.91 per share.

From a technical standpoint, this stock has been annihilated during the last three months after it fell from a high of $35.60 in late May to its current price near $13.50 a share. Shares of RLD are now showing an extreme oversold reading measured by the relative strength index, which stands at around 23. This low of an RSI reading often indicates a point from which a stock can see a sharp bounce higher.

If you want to get long RLD for a near-term bounce play, then I would buy this on any weakness and simply stop out if it trades below its most recent support zone of $13.27 a share. I would add pretty aggressively to any long position if you see RLD move above $16 a share since there's a nice gap up to $19 that could get filled quickly. I would consider shorting this name if it falls below $13.27 on big volume. The three-month average volume on RLD is 1.3 million shares, so use that to key off of any long or short trade.

It's worth pointing out that RLD is one heavily shorted stock by the bears. The current short interest as a percentage of the float for RLD stands at an enormous 34.7%. This stock could easily see a nice short squeeze if the bulls decide to move back into it in the coming weeks or months, so keep this name on your radar.

To see more stocks with notable insider buying, including Overstock.com ( OSTK), Unisys ( UIS) and Cincinnati Financial ( CINF), check out the Stocks With Big Insider Buying portfolio on Stockpickr.

-- Written by Roberto Pedone in Winderemere, Fla.

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At the time of publication, author had no positions in stocks mentioned.

Roberto Pedone, based out of Windermere, Fla., is an independent trader who focuses on stocks, options, futures, commodities and currencies. He is also an outside contributor to Beconequity.com and maintains the website Maddmoney.net, which he sold to Blue Wave Advisors in 2008. Roberto studied International Business at The Milwaukee School of Engineering, and he spent a year overseas studying business in Lubeck, Germany.