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.

Stocks with notable insider activity is something that I tweet about on a regular basis. These are also the exact type of stocks that I love to trade and alert in real-time.

At the end of the day, it's 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 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.

Opko Health

One biopharmaceutical and diagnostics player that insiders are active in here is Opko Health  (OPK - Get Report) , which engages in the discovery, development and commercialization of novel and proprietary technologies in the U.S., Ireland, Chile, Spain, Israel and Mexico. Insiders are buying this stock into decent strength, since shares have risen by 15.7% over the last six months.

Opko Health has a market cap of $5.6 billion and an enterprise value of $5.9 billion. This stock trades at a fair valuation, with a forward price-to-earnings of 60.4. Its estimated growth rate for this year is 16.7%, and for next year it's pegged at 440%. This is a cash-rich company, since the total cash position on its balance sheet is $193.60 million and its total debt is $49.40 million.

The CEO just bought 48,600 shares, or about $500,000 worth of stock, at $10.23 to $10.43 per share.

From a technical perspective, Opko Health is currently trending above its 50-day moving average and just below its 200-day moving average, which is neutral trendwise. This stock recently pulled back off its high of $11.85 a share to around $9.83 a share. Following that drop, shares of Opko Health have now started to bounce off that $9.83 area, and it's beginning to move within range of triggering a near-term breakout trade.

If you're bullish on Opko Health then I would look for long-biased trades as long as this stock is trending above some near-term support at $9.83 a share or above its 50-day moving average of $9.36 a share and then once it breaks out above its 20-day moving average of $10.58 a share to some more near-term resistance at $10.62 a share with high volume. Look for a sustained move or close above those levels with volume that hits near or above its three-month average action of 4.51 million shares. If that breakout triggers soon, then this stock will set up to re-test or possibly take out its next major overhead resistance levels at $11.75 to $11.85, or even $13 a share.

Conn's

Another electronics stores player that insiders are in love with here is Conn's  (CONN - Get Report) , which operates as a specialty retailer of durable consumer goods and related services in the U.S. Insiders are buying this stock into massive weakness, since shares have dropped sharply by 53.2% over the last six months.

Conn's has a market cap of $352 million and an enterprise value of $1.4 billion. This stock trades at a cheap valuation, with a trailing price-to-earnings of 9.3 and a forward price-to-earnings of 7. Its estimated growth rate for this year 14.6%, and for next year it's pegged at 37.3%. This is not a cash-rich company, since the total cash position on its balance sheet is $109.07 million and its total debt is $1.16 billion.

A beneficial owner just bought 250,000 shares, or about $3.08 million worth of stock, at $12.35 per share.

From a technical perspective, Conn's is currently trending below both its 50-day and 200-day moving averages, which is bearish. This stock has been downtrending badly over the last month and change, with shares moving lower off its high of $21.48 a share to its new 52-week low hit on Tuesday of $11.36 a share. During that downtrend, shares of Conn's have been consistently making lower highs and lower lows, which is bearish technical price action.

If you're in the bull camp on Conn's, then I would look for long-biased trades as long as this stock is trending above its new 52-week low of $11.36 a share and then once it breaks out above some near-term overhead resistance levels at $12.64 to around $13 a share with volume that hits near or above its three-month average action of 1.03 million shares. If that breakout materializes soon, then this stock will set up to re-test or possibly take out its next major overhead resistance levels at $15 to its 50-day moving average of $15.41 a share.

Mattress Firm

One specialty retailer that insiders are loading up on here is Mattress Firm  (MFRM) , which operates as a retailer of mattresses and related products and accessories in the U.S. Insiders are buying this stock into modest weakness, since shares have fallen by 6.7% over the last three months.

Mattress Firm has a market cap of $1.5 billion and an enterprise value of $2.1 billion. This stock trades at a reasonable valuation, with a trailing price-to-earnings of 25.7 and a forward price-to-earnings of 13.2. Its estimated growth rate for this year is 7.2%, and for next year it's pegged at 24.9%. This is not a cash-rich company, since the total cash position on its balance sheet is $10.28 million and its total debt is $697.50 million.

A beneficial owner just bought 104,134 shares, or about $4.41 million worth of stock, at $42.44 per share.

From a technical perspective, Mattress Firm is currently trending above its 50-day moving average and below its 200-day moving averages, which is neutral trendwise. This stock has been trending sideways over the last few weeks, with shares moving between just below $40 on the downside and $43.40 on the upside. Any high-volume move above that recent range high soon could easily trigger a near-term breakout trade.

If you're bullish on Mattress Firm, then I would look for long-biased trades as long as this stock is trending above its 50-day moving average of $40.42 a share and then once it breaks out above some near-term overhead resistance levels at $43 to $43.40 a share and then over $44.50 a share with high volume. Look for a sustained move or close above those levels with volume that hits near or above its three-month average action of 481,274 shares. If that breakout triggers soon, then this stock will set up to re-test or possibly take out its next major overhead resistance levels at $47.50 to its 200-day moving average of $48.63, or even $50 to $52 a share.

Clayton Williams Energy

One energy player that insiders are jumping into here is Clayton Williams Energy  (CWEI) , which explores for and develops and produces oil and natural gas primarily in Texas and New Mexico Insiders are buying this stock into massive weakness, since shares have plunged by 81.4% over the last six months.

Clayton Williams Energy has a market cap of $116 million and an enterprise value of $852 million. This stock trades at a cheap valuation, with a price-to-sales of 0.40 and a price-to-book of 0.32. Its estimated growth rate for this year is -17.1%, and for next year it's pegged at 24.3%. This is not a cash-rich company, since the total cash position on its balance sheet is $8.74 million and its total debt is $749.74 million.

A director just bought 287,000 shares, or about $2.52 million worth of stock, at $8.65 to $9.12 per share.

From a technical perspective, Clayton Williams Energy is currently trending below both its 50-day and 200-day moving averages, which is bearish. This stock has been uptrending over the last few weeks, with shares moving higher off its new 52-week low of $6.35 a share to its intraday high on Tuesday of $9.69 a share. During that uptrend, shares of Clayton Williams Energy have been making mostly higher lows and higher highs, which is bullish technical price action.

If you're bullish on Clayton Williams Energy, then I would look for long-biased trades as long as this stock is trending above some near-term support levels at around $8.50 to $8 a share and then once it breaks out above some near-term overhead resistance levels at $10 to its 20-day moving average of $10.02 a share with volume that hits near or above its three-month average action of 449,742 shares. If that breakout takes hold soon, then this stock will set up to re-test or possibly take out its next major overhead resistance levels at $11.40 to $12, or even $13 a share.

Itron

One final stock with some decent insider buying is technology player Itron  (ITRI - Get Report) , which provides metering solutions to electricity, gas and water utility markets worldwide. Insiders are buying this stock into strength, since shares jumped by 21.9% over the last six months.

Itron has a market cap of $1.5 billion and an enterprise value of $1.8 billion. This stock trades at a fair valuation, with a trailing price-to-earnings of 63 and a forward price-to-earnings of 16.9. Its estimated growth rate for this year is 107.9%, and for next year it's pegged at 13.8%. This is not a cash-rich company, since the total cash position on its balance sheet is $131.02 million and its total debt is $371.21 million.

A beneficial owner just bought 62,710 shares, or about $2.62 million worth of stock at $41.81 per share. From a technical perspective, Itron is currently trending above both its 50-day and 200-day moving averages, which is bullish. This stock has been uptrending strong over the last three months, with shares moving higher off its low of $29.02 a share to its recent high of $43.29 a share. During that uptrend, shares of Itron has been making mostly higher lows and higher highs, which is bullish technical price action.

If you're bullish on Itron, then I would look for long-biased trades as long as this stock is trending above some near-term support levels at $39.20 a share to its 50-day moving average of $38.56 a share and then once it breaks out above some near-term overhead resistance levels at its 20-day moving average of $41.61 a share to $42.24 a share and then over its 52-week high of $43.29 a share with high volume. Look for a sustained move or close above those levels with volume that hits near or above its three-month average volume of 330,000 shares. If that breakout develops soon, then this stock will set up to enter new 52-week-high territory, which is bullish technical price action.

Disclosure: This article is commentary by an independent contributor. At the time of publication, the author held no positions in the stocks mentioned.