As hard as it may seem to find good stocks to buy, the selling process is far trickier, so much so that the ability to know what — and when — to sell is arguably what separates winning investors from the also-rans.

Why is selling so hard? In a word, psychology. You may buy a stock because you think it's cheap or because you love the company's products. But once you own it, you not only acquire a vested interest in making money but you also want to feel good about your decision.

The key to becoming a good seller is to take emotion out of the equation. Simply ask yourself, "Would I buy this stock today?" If the answer is no, then sell it. In fact, there are plenty of reasons to sell.

One of the simplest is to sell when a stock becomes overpriced. For many investors, the most familiar measure of value is a stock's price-earnings ratio. The P/E essentially tells how much an investor pays for each dollar per share a company earns. You can study P/Es based on how much per share a company earned in the previous year or on the basis of projected earnings, usually for the current year or for the next 12 months.

Whatever approach you take, you must be consistent in the earnings you use. You can compare a stock's P/E with that of the overall market — currently 34 for Standard & Poor's 500-stock index, based on the past year's earnings, and 18 on 2003 estimated profits. Using S&P reports, you can compare a stock's P/E with the average P/E for its industry. Or you can compare a company's P/E with its past.

Sometimes a stock's price is simply beyond the pale. A perfect example is Amazon.com, the online retailer. At a recent price of $30, Amazon (symbol AMZN) sells at 63 times the 48 cents per share that analysts surveyed by Thomson First Call estimate it will earn in 2003. Even if profits grow 25 percent annually over the next few years, as analysts predict, it will take a little more than three years for the company to earn $1 per share, which would make its P/E a more reasonable 30. Two other Web stocks, eBay (EBAY) and Yahoo! (YHOO), are in similarly rarefied territory. If you own any of these three, now may be a good time to sell.