BOSTON (CNN/Money) - eBay accomplished something it hadn't yet done in its roughly seven years as a public company -- something its investors hope it never does again.

It missed the Wall Street analysts' estimates.

As a result the stock dropped almost 20 percent, erasing the gains it had made since last September.

Now that the dust has mostly settled, let's assess what this means for investors and explore various ways to play this clothesless emperor.

And that's what this is, really: the first recognition that eBay -- long thought to be immortal and the one Net stock that you had to own even if you hated Net stocks -- is, in fact, a mortal company that must adhere to the laws of stock market gravity.

That brings up the first lesson from last week: Buyer beware.

Any stock that's trading with a forward price/earnings ratio of almost 67, as eBay was earlier last week, is already so frothy that even a slight misstep will send it crashing. eBay didn't miss by much -- only a penny. But it was a costly penny, which wiped out almost $13 billion of the company's value.

Of course, the miss was only partly responsible for the free fall. More troubling to some observers was the announcement that the company would have to spend more on sales and marketing to keep its customer base growing.

Mark Mahaney, an analyst with American Technology Research, says that news surprised him. "It's tough to get a read on operating expenses in a quarter," he says. "There weren't expectations that there'd be a big sales and marketing increase."

Mahaney's postearnings eBay note is the first analyst note I've ever read that includes the word "sucks."

I can't help but notice that the only good news in eBay's increased sales and marketing expenditures is for Google or Yahoo. "Google is one of eBay's largest sources of registrations," says Youssef Squali, an analyst with Jefferies & Co. "Search advertising is a big driver to eBay."

One tempting school of thought is to regard eBay's (Research) recent stock movement as a 20-percent-off sale. Indeed, Legg Mason's Scott Devitt is urging clients to pick up some shares on the drop. He issued an upgrade to "buy" on Friday morning, with a $100 price target.

"There was an overreaction," he told me. "There were six analyst downgrades on Thursday. The company is still as dominant as it was the day before yesterday, before it missed by a penny. The long-term story didn't change."

But many analysts caution that the company is still overpriced, with a diminished sheen on the Street.

Martin Pyykkonen, an analyst with Janco Research, has a target price of $90 for eBay, a little north of the Friday close of $86.05.

The company's 2005 earnings-per-share estimates are between $1.48 and $1.52. To justify a $90-a-share price, eBay would have to increase its earnings two and a half times this year. Everyone knows that that isn't going to happen, and Pyykkonen notes that many technology companies trade at about twice their earnings growth rates.

"You really need to see further earnings acceleration or see the stock come down," he warns. Pyykkonen has a "neutral" rating on the stock. ("Neutral" usually means "sell" on Wall Street.)

If you're still planning to jump into eBay on this drop, you'll want to pay attention to some metrics that have taken on heightened relevance. Acquiring customers -- and at eBay the customers are the sellers, not the buyers -- is expensive, so you should keep track of the number of eBay power sellers. The high-volume vendors will generate a higher ratio of fees to acquisition cost.

Second, you'll want to keep an eye on international growth. (See the story on eBay's international efforts) in the January/February issue of this magazine.) eBay's success abroad is hardly guaranteed, as evidenced by the company's ceding the Japanese market to Yahoo a few years ago, but this could be its salvation.

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