The hope of profits and the joy of ownership make buying stocks a fairly simple decision, especially in comparison to the tormented hair-pulling that's often associated with selling.

When to jettison a stock is a difficult decision, so we won't pretend there's a one-size-fits-all formula. However, guidelines can make selling decisions easier. At Motley Fool Pro, the following are four key factors in any sell consideration.

1. Valuation
The most cited reason to sell -- a fairly valued stock -- is also the most difficult to nail down.

We estimate the fair value of a company before plunking money down to buy, determining intrinsic value by digging into financial statements, analyzing business prospects and free cash flow, and making conservative assumptions about future growth.

Buying undervalued stocks, we wait patiently for a price that's close to our estimate of fair value, reassess at that point, and then ruthlessly sell if the stock looks fairly priced. One challenge is overcoming your attachment to the stock -- everyone loves a winner -- and the dream that it will go up indefinitely. You may love everything about Wynn Resorts (Nasdaq: WYNN), but at more than 100 times earnings estimates, the valuation says sell anyway.

That said, decisions based just on valuation can mislead you sometimes. Netflix (Nasdaq: NFLX) has looked rather expensive for years, but continues to reward shareholders. If valuation on a leading business is perplexing you, you may consider selling just some of your shares (to lock in profits) or protecting your gains through other means (including options), and then consider other factors that may influence a sell decision.

2. Fundamental change in the underlying business
Companies are always undergoing change -- sometimes for the better, oftentimes not. As patient investors, we're willing to tolerate minor, fixable hiccups along the lines of a weak quarter or delayed product launch.

We're not so forgiving of major blunders -- think acquisitions that undermine the core business, getting surpassed by a competitor, taking on unproductive debt, or a string of failed new product attempts. Whenever a business undergoes a significant change, you need to put on your thinking cap and reassess.

Palm, Inc. (Nasdaq: PALM) has underdone organizational changes and management shakeups, lost its early lead in handheld devices, and taken on debt, all in the past handful of years. Not surprisingly, it hasn't created any shareholder value during this time.

3. Challenges to your investing thesis
When you make a buy decision, you should write down your reasons and keep them handy. Knowing the most important drivers behind your buys, you can reassess your decision if any part of your thesis is challenged.

Because valuation is part of any thesis, threatening changes can include dividend cuts, deterioration of margins, weakening free cash flow --- or economic shifts. At Pro, we keep the big picture in mind. If you'd bought Lowe's (NYSE: LOW) or Best Buy (NYSE: BBY) before 2007, believing a housing boom would continue, you'd have followed housing news closely and may have seen your thesis falling apart -- forcing a timely sale. With valuations having fallen and growth resuming, they look like much better bets today.

So, what's the thesis behind each stock you own? Write it down.

4. Better places for your money
Sometimes a sell decision has little to do with the holding itself -- you may simply see better opportunities elsewhere and lack the funds to take advantage. Just as a soccer coach will swap tired players for fresh ones in order to win the game, your portfolio can benefit from shuffling some players, too.

In the late 1990s, it was becoming apparent that Johnson & Johnson (NYSE: JNJ) had a better business model than the pure drug companies, including Merck (NYSE: MRK). Since 1999, J&J has gained about 50% while Merck has lost 50%. That was a great swap. I still think Johnson & Johnson's diversity provides more stability than drug-only competitors.

Just like the five traits of great stocks we keep in mind when we buy, these are some of the criteria at the forefront of our sell decisions at Motley Fool Pro. To keep membership manageable, Motley Fool Pro will open for only a few days this month, the first time members have been able to join since January. Join us while you can -- and let us sweat out the tough sell decisions for you. You can get more information about Motley Fool Pro by entering your email in the box below.

Jeff Fischer is the advisor of Motley Fool Pro. He doesn't own shares of any companies mentioned. Best Buy and Lowe's are Motley Fool Inside Value recommendations. Best Buy and Netflix are Stock Advisor selections. Johnson & Johnson is an Income Investor choice. Motley Fool Options has recommended a bull call spread position on Best Buy and buying calls on Johnson & Johnson. The Fool owns shares of Best Buy. The Motley Fool has a disclosure policy.