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.

Wintrust Financial

One financial player that insiders are loading up on here is Wintrust Financial  (WTFC - Get Report) , which provides banking and other financial products and services to customers in the Chicago metropolitan area and in southern Wisconsin. Insiders are buying this stock into big strength, since shares have soared higher by 33.5% over the last six months.

Wintrust Financial has a market cap of $2.7 billion and an enterprise value of $3 billion. This stock trades at a fair valuation, with a trailing price-to-earnings of 17 and a forward price-to-earnings of 14.2. Its estimated growth rate for this year is 21.5%, and for next year it's pegged at 6.2%. This is not a cash-rich company, since the total cash position on its balance sheet is $968.46 million and its total debt is $1.23 billion.

A director just bought 9,500 shares, or about $504,000 worth of stock at $53.11 per share. From a technical perspective, Wintrust Financial is currently trending above both its 50-day and 200-day moving averages, which is bullish. This stock has been uptrending over the last month and change, with shares moving higher off its low of $46.62 a share to its recent high of $54 a share. During that uptrend, shares of Wintrust Financial have been making mostly higher lows and higher highs, which is bullish technical price action. That move has now pushed this stock within range of triggering a big breakout trade above some key near-term overhead resistance levels.

If you're bullish on Wintrust Financial, then I would look for long-biased trades as long as this stock is trending above its 50-day moving average of $51.63 a share and then once it breaks out above some near-term overhead resistance levels at $54 to $54.09 a share and then above its 52-week high of $55 a share with high volume. Look for a sustained move or close above those levels with volume that registers near or above its three-month average volume of 362,897 shares. If that breakout fires off soon, then this stock will set up to enter new 52-week-high territory, which is bullish technical price action. Some possible upside targets off that move are $60 to $65 a share.

Delta Air Lines

Another stock that insiders are active in here is Delta Air Lines  (DAL - Get Report) , which provides scheduled air transportation for passengers and cargo in the U.S. and internationally. Insiders are buying this stock into notable weakness, since shares have fallen by 13.6% over the last six months.

Delta Air Lines has a market cap of $28.8 billion and an enterprise value of $34.4 billion. This stock trades at a cheap valuation, with a trailing price-to-earnings of 6.2 and a forward price-to-earnings of 6.5. Its estimated growth rate for this year 27.1%, and for next year it's pegged at 1.2%. This is not a cash-rich company, since the total cash position on its balance sheet is $2.96 billion and its total debt is $7.80 billion. This stock currently sports a dividend yield of 1.3%.

The CFO just bought 25,000 shares, or about $972,000 worth of stock, at $33.91 per share.

From a technical perspective, Delta Air Lines is currently trending below both its 50-day and 200-day moving averages, which is bearish. This stock has been uptrending over the last month and change, with shares moving higher off its low of $32.60 a share to its recent high of $41.35 a share. During that uptrend, shares of Delta Air Lines have been making mostly higher lows and higher highs, which is bullish technical price action.

If you're bullish on Delta Air Lines then I would look for long-biased trades as long as this stock is trending above some near-term support levels at $38.27 or at $37 a share and then once it breaks out above some near-term overhead resistance levels at $39.20 to its 50-day moving average of $39.80 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 11.81 million 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 $41.35 to $43, or even $44.50 to its 200-day moving average of $45.36 a share.

Netflix

One Internet TV network player that insiders are jumping into here is Netflix  (NFLX - Get Report) , which engages in the Internet delivery of TV shows and movies on various Internet-connected screens. Insiders are buying this stock into modest weakness, since shares are off by 1% over the last six months.

Netflix has a market cap of $38.7 billion and an enterprise value of $39.7 billion. This stock trades at a premium valuation, with a trailing price-to-earnings of 282 and a forward price-to-earnings of 101. Its estimated growth rate for this year 0%, and for next year it's pegged at 217.9%. This is not a cash-rich company, since the total cash position on its balance sheet is $1.83 billion and its total debt is $2.37 billion.

A director just bought 600,000 shares, or about $51.85 million worth of stock, at $85.76 to $87.60 per share.

From a technical perspective, Netflix is currently trending below both its 50-day and 200-day moving averages, which is bearish. This stock recently gapped-down sharply lower from around $100 a share to 84.50 a share with heavy downside volume flows. Following that move, shares of Netflix then gapped-up from around $84.50 to $93.10 a share with decent upside volume flows. This second move has now pushed shares of Netflix within range of triggering a big breakout trade above some key near-term overhead resistance levels.

If you're in the bull camp on Netflix, then I would look for long-biased trades as long as this stock is trending above some near-term support at $90 a share or around $88 a share and then once it breaks out above some near-term overhead levels at $93.10 to its 50-day moving average of $94.33 a share with volume that hits near or above its three-month average action of 12.99 million shares. If that breakout triggers soon, then this stock will set up to re-fill some of its previous gap-down-day zone from July that started near $100 a share.

Bank of New York Mellon

One asset management player that insiders are loading up on here is Bank of New York Mellon  (BK - Get Report) , which provides financial products and services to institutions, corporations and high-net-worth individuals in the U.S. and internationally. Insiders are buying this stock into notable strength, since shares have trended up 24.5% over the last six months.

Bank of New York Mellon has a market cap of $43 billion and an enterprise value of -$47.6 billion. This stock trades at a fair valuation, with a trailing price-to-earnings of 14.3 and a forward price-to-earnings of 11.9. Its estimated growth rate for this year is 6.7%, and for next year it's pegged at 9.9%. This is a cash-rich company, since the total cash position on its balance sheet is $143.36 billion and its total debt is $53.45 billion. This stock currently sports a dividend yield of 5.5%.

A director just bought 152,000 shares, or about $5.90 million worth of stock, at $33.87 per share.

From a technical perspective, Bank of New York Mellon is currently trending above both its 50-day and 200-day moving averages, which is bullish. This stock has been uptrending over the last month or so, with shares moving higher off its low of $35.72 a share to its recent high of $40.67 a share. During that uptrend, shares of Bank of New York Mellon have been making mostly higher lows and higher highs, which is bullish technical price action. That move has now pushed this stock within range of triggering a near-term breakout trade.

If you're bullish on Bank of New York Mellon, then I would look for long-biased trades as long as this stock is trending above its 20-day moving average of $38.92 a share and then once it breaks out above some near-term overhead resistance levels at $40.30 to $40.67 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 5.44 million shares. If that breakout gets started soon, then this stock will set up to re-test or possibly take out its next major overhead resistance levels at $41.50 to $42.60, or even $44 to its 52-week high of $45.45 a share.

Mattress Firm

My final stock with some decent insider buying is specialty retailer Mattress Firm  (MFRM) , which operates as a specialty retailer of mattresses and related products and accessories in the U.S. Insiders are buying this stock into notable weakness, since shares have fallen sharply by 22% over the last six months.

Mattress Firm a market cap of $1.1 billion and an enterprise value of $2.5 billion. This stock trades at reasonable valuation, with a forward price-to-earnings of 10.6. Its estimated growth rate for this year is -5.1%, and for next year it's pegged at 24.6%. This is not a cash-rich company, since the total cash position on its balance sheet is $1.14 million and its total debt is $1.47 billion.

A beneficial owner just bought 78,310 shares, or about $2.39 million worth of stock, at $30.64 per share.

From a technical perspective, Mattress Firm is currently trending below both its 50-day and 200-day moving averages, which is bearish. This stock has been downtrending over the last two months, with shares moving lower off its high of $35.75 a share to its recent low of $29.56 a share. During that downtrend, shares of Mattress Firm have been making mostly lower highs and lower lows, which is bearish technical price action.

If you're bullish on Mattress Firm, then I would look for long-biased trades as long as this stock is trending above its recent low of $29.56 a share or above $29 a share and then once it breaks out above some near-term overhead resistance levels at $31 to its 20-day moving average of $31.74 a share and its 50-day moving average of $32.40 a share with volume that hits near or above its three-month average action of 376,756 shares. If that breakout develops soon, then this stock will set up to re-test or possibly take out its next major overhead resistance levels at $34 to $35.75, or even $37 to $38 a share.

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