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.

Western Refining

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One energy player that insiders are active in here is Western Refining (WNR) , which operates as an independent crude oil refiner and marketer of refined products. Insiders are buying this stock into big weakness, since shares have fallen by 53% over the last six months.

Western Refining has a market cap of $1.9 billion and an enterprise value of $2.9 billion. This stock trades at a cheap valuation, with a trailing price-to-earnings of 5.1 and a forward price-to-earnings of 9.7. Its estimated growth rate for this year is -59%, and for next year it's pegged at 11%. This is not a cash-rich company, since the total cash position on its balance sheet is $709.57 million and its total debt is $1.60 billion. This stock currently sports a dividend yield of 6.9%.

The CEO just bought 150,000 shares, or about $3.30 million worth of stock at $20.95 per share. From a technical perspective, Western Refining is currently trending below both its 50-day and 200-day moving averages, which is bearish. This stock has been downtrending over the last few weeks, with shares falling sharply off its high of $29.45 a share to its recent low of $20.64 a share. During that slide, this stock has been consistently making lower highs and lower lows, which is bearish technical price action. That said, this stock has now started to bounce a bit off that $20.64 low and off oversold levels, since its current relative strength index reading is 34.9.

If you're bullish on Western Refining, then I would look for long-biased trades as long as this stock is trending above its new 52-week low of $20.64 a share and then once it breaks out above some near-term overhead resistance levels at $23 to $23.50 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 volume of 2.71 million shares. If that breakout fires off soon, then this stock will set up to re-fill some of its previous gap-down-day zone that started near $15.71 a share.

Mirati Therapeutics

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A clinical-stage biopharmaceutical player that insiders are jumping into here is Mirati Therapeutics (MRTX) - Get Mirati Therapeutics Inc. Report , which develops a pipeline of products targeting oncology. Insiders are buying this stock into large weakness, since shares have plunged by 62.5% over the last six months.

Mirati Therapeutics has a market cap of $309 million and an enterprise value of $167 million. This stock trades at a reasonable valuation, with price-to-book of 2.28. Its estimated growth rate for this year is -17.3%, and for next year it's pegged at -0.20%. This is a cash-rich company, since the total cash position on its balance sheet is $138.01 million and its total debt is zero.

A beneficial owner just bought 65,782 shares, or about $1.19 million worth of stock, at $17.15 per share.

From a technical perspective, Mirati Therapeutics is currently trending below both its 50-day and 200-day moving averages, which is bearish. This stock has been downtrending over the last month and change, with shares falling off its high of $24.43 a share to its new 52-week low of $14.59 a share. During that downtrend, shares of Mirati Therapeutics have been consistently making lower highs and lower lows, which is bearish technical price action.

If you're bullish on Mirati Therapeutics then I would look for long-biased trades as long as this stock is trending above its new 52-week low of $14.59 a share and then once it breaks out above some near-term overhead resistance levels at $16 to $18 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 action of 178,482 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 its 20-day moving average of $20.44 to its 50-day moving average of $21.09 a share, or even $22 a share.

Greenlight Capital

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One financial player that insiders are active in here is Greenlight Capital (GLRE) - Get Greenlight Capital Re, Ltd. Class A Report , which engages in the provision of property and casualty reinsurance products and services worldwide. Insiders are buying this stock into notable strength, since shares have jumped higher by 10.3% over the last three months.

Greenlight Capital has a market cap of $796 million and an enterprise value of -$261 million. This stock trades at a fair valuation, with a forward price-to-earnings of 5.8. Its estimated growth rate for this year 136.9%, and for next year it's pegged at 12.8%. This is a cash-rich company, since the total cash position on its balance sheet is $1.06 billion and its total debt is zero.

An officer just bought 40,000 shares, or about $840,000 worth of stock, at $20.99 per share.

From a technical perspective, Greenlight Capital is currently trending below both its 50-day and 200-day moving averages, which is bearish. This stock has been trending sideways and consolidating over the last two months, with shares moving between $20.48 on the downside and $22.13 on the upside. Any high-volume move above the upper-end of its recent sideways trending chart pattern for shares of Greenlight Capital could trigger a big breakout trade.

If you're in the bull camp on Greenlight Capital, then I would look for long-biased trades as long as this stock is trending above some key near-term support levels at $20.89 to $20.48 a share and then once it breaks out above its 200-day moving average of $21.77 a share to some more key resistance levels at $22.07 to $22.13 a share with volume that hits near or above its three-month average action of 141,248 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 $22.50 to $24, or even $25 to $26 a share.

Hyatt Hotels

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One lodging player that insiders are loading up on here is Hyatt Hotels (H) - Get Hyatt Hotels Corporation Class A Report , which owns, operates, manages, franchises, licenses or provides services to full and select service hotels, resorts and residential and vacation properties. Insiders are buying this stock into major strength, since shares have ripped higher by 31.4% over the last three months.

Hyatt Hotels has a market cap of $6.7 billion and an enterprise value of $7.7 billion. This stock trades at a premium valuation, with a trailing price-to-earnings 58.4 and a forward price-to-earnings of 31.1. Its estimated growth rate for this year is 52.2%, and for next year it's pegged a 17.5%. This is not a cash-rich company, since the total cash position on its balance sheet is $503 million and its total debt is $1.38 billion.

An officer just bought 25,000 shares, or about $1.22 million worth of stock, at $48.98 per share.

From a technical perspective, Hyatt Hotels is currently trending above both its 50-day and 200-day moving averages, which is bullish. This stock has been consolidating and trending sideways over the last three months, with shares moving between $44.76 on the downside and $50.94 on the upside. Shares of Hyatt Hotels have recently started to spike higher off all of its key moving averages, and it's now quickly moving within range of triggering a near-term breakout trade.

If you're bullish in Hyatt Hotels, then I would look for long-biased trades as long as this stock is trending above some near-term support at $48.16 a share or above more support at $46 a share and then once it breaks out above some near-term overhead resistance at $50.94 a share with high volume. Look for a sustained move or close above that level with volume that hits near or above its three-month average action of 641,760 shares. If that breakout kicks off soon, then this stock will set up to re-test or possibly take out its next major overhead resistance levels at $54 to $58 a share.

Mindbody

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My final stock with some notable insider buying is technology player Mindbody (MB) - Get MINDBODY, Inc. Class A Report , which operates a cloud-based business management software and payments platform for the small and medium-sized businesses in the wellness services industry. Insiders are buying this stock into strength, since shares have risen by 38.6% over the last three months.

Mindbody has a market cap of $558 million and an enterprise value of $487 million. This stock trades at a reasonable valuation, with a price-to-sales of 5.57 and a price-to-book of 5.34. Its estimated growth rate for this year is 29.9%, and for next year it's pegged at 55.6%. This is a cash-rich company, since the total cash position on its balance sheet is $93.40 million and its total debt is $15.96 million.

A beneficial owner just bought 296,794 shares, or about $4.43 million worth of stock, at $14.35 to $14.79 per share.

From a technical perspective, Mindbody is currently trending above both its 50-day and 200-day moving averages, which is bullish. This stock has been uptrending over the last three months, with shares moving higher off its low of $9.20 a share to its recent high of $15.50 a share. During that uptrend, this stock has been making mostly higher lows and higher highs, which is bullish technical price action. That strong move higher has now pushed shares of Mindbody within range of triggering a near-term breakout trade.

If you're bullish on Mindbody, then I would look for long-biased trades as long as this stock is trending above its 50-day moving average of $13.65 a share and then once it breaks out above some near-term overhead resistance levels at $15 to $15.50 a share and then above more key resistance levels at $15.91 to $16.27 a share with volume that hits near or above its three-month average action of 156,026 shares. If that breakout fires off soon, then this stock will set up to re-test or possibly take out its next major overhead resistance levels at $18 to its 52-week high of $18.39, or even $19 to $20 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.