(FORTUNE Magazine) – Buying on the dips is a favorite--if risky--technique of investment pros and amateurs alike. And when eBay (EBAY, $78) announced its first-ever earnings shortfall (by a penny) on Jan. 19 and lowered future expectations, it took one doozy of a dip. The online auctioneer's share price fell 19% the next day, wiping out a cool $13 billion in market value. It has since shed another $3.5 billion. Does that make this a good entry point for investors who missed out on the stock's past gains?

Valuing eBay always has been more art than science. But it is instructive to look at what investors historically have been willing to pay for eBay's future earnings. An analysis of First Call profit projections shows that eBay's forward price/earnings ratio has been coming down steadily even as the stock price skyrocketed. After peaking above 1,800 in 1999 and averaging close to 100 in 2001, the forward P/E has averaged about 61 over the past three years. Today it stands at 52. The last time the forward multiple got that low was in September 2002, when the stock sold for about $30 on a split-adjusted basis. Still, it's hard to think of eBay as undervalued now. Shares are trading at more than 15 times sales. And the P/E of 52 matches that of Google, which is growing three times as fast. Better to sit this auction out. -- Adam Lashinsky