If they knew what a perfect storm is brewing on the horizon, maybe they wouldn't bid Amazon's stock to almost $300. At $300 a share, Amazon has a forward earnings multiple of over a 100 by some estimates. The sky-high, oxygen-tank-requiring earnings multiple is based primarily on the view revenue and earnings will continue growing. That belief could shatter immediately upon the entry of an old, well-established market leading participant that is new to the North American market. Remember, not only is Alibaba the leader in e-commerce in China, it is also the place from which many sellers on Amazon and eBay already source their products. It's a small step for existing American online retailers to move from Alibaba sourcing to listing products on "Alibaba U.S.A." This becomes even more apparent when you consider Alibaba charges customers less than Amazon or eBay. I bet Alibaba is finalizing details (if it hasn't already) on how existing wholesale buyers become retail sellers with the push of a button. Alibaba already has everything in place mechanically to make it happen. Alibaba has a payment processor similar to PayPal and Amazon Payments called Alipay. Alipay claims more active customers than PayPal and Amazon combined. For Yahoo!, it's a mixed bag. Alibaba's entry into North America represents increased competition, but since Yahoo! owns about 20% of Alibaba, Yahoo! shares could double in value. Yahoo!'s market cap based on Tuesday's close is about $29 billion. If Alibaba IPOs and successfully establishes a leading presence in North America, investors may value the Chinese company at $100 billion or more. The prognosis for Amazon upon an Alibaba entry isn't quite as ameliorative. If investors no longer believe Amazon will continue growing rapidly, the shares may trade at a fraction of the current price. eBay trades at a forward earnings multiple of 17, Apple ( AAPL) at 10 and Cisco Systems ( CSCO) near 12. If you assign a forward earnings multiple of 20 to Amazon, the shares should trade near $65. Of course, for Amazon to trade near $65 after Alibaba enters the market, you have to believe Amazon will continue trading at a higher multiple than the other companies. If Amazon is losing money and marketshare, $65 may turn into little more than wishful thinking by investors remembering days gone by.