NEW YORK (CNN/Money) - Amazon and eBay have never been the best of friends. Some say it's sour grapes: Amazon held the title of Internet poster child for a long time, until its grand ambitions caused it to lose focus on its core business. eBay, on the other hand, has largely continued to meet or beat estimates, while also expanding smartly.

Amazon (AMZN: down $0.43 to $18.33, Research, Estimates) is creeping back into favor, however, posting its first-ever profits, raising financial estimates, and seeing its stock rise from a low of $5.97 on Sept. 28, 2001, to its current price of about $19. Even analysts are hopping on the Amazon train, with 18 of the 19 firms that cover the stock increasing their estimates in the last four weeks.

One of those analysts, Safa Rashtchy of U.S. Bancorp Piper Jaffray, believes that Amazon is ready to make a radical shift in its marketing strategy -- one that edges it closer to eBay's (EBAY: down $0.93 to $54.21, Research, Estimates) territory. In a report filed May 14, Rashtchy writes that his research and communications with Amazon indicate that it is prepared to start selling items not currently offered in its Marketplace area -- where new, used, and refurbished items are sold alongside brand new products -- a move that could "potentially impact some of eBay's listings." Rashtchy expects the company to launch these initiatives this summer.

Currently, Amazon allows used listings only for products it also sells retail. The new feature would allow anyone -- from individuals to small businesses -- to sell used items on Amazon's product pages, regardless of whether Amazon sells the item retail. For these listings, Amazon would process the payments and collect fees and commissions.

Sound like a minor change? It's not. The Marketplace section accounted for 23 percent of Amazon's total U.S. sales for the first quarter of 2002, up from 4 percent a year earlier. "That's very impressive," says Ken Cassar, senior analyst at Jupiter Media Metrix (JMXI: down $0.01 to $0.27, Research, Estimates), "considering they've only been doing it for a year and a half." Allowing anyone to create unique product pages and descriptions should boost that number significantly. What's more, Marketplace revenues are almost pure profit, since Amazon manages no inventory and handles no shipping in the transactions. Sound familiar? It should: That model is eBay's bread and butter.

This isn't the first time Amazon has tried to borrow a page from the eBay playbook. Amazon started allowing auctions in March of 1999, but the auctions tab no longer sits at the top of the homepage, and the company remains a very distant second behind eBay in auction volume and revenue.

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In conversation, Rashtchy stresses that Amazon's move is only a potential threat to eBay. And Jupiter's Cassar thinks it will be a long time -- if ever -- before Amazon's Marketplace initiatives pose any serious challenge to eBay's dominance. "The person-to-person auction market may naturally be a monopoly market," Cassar says, "and eBay may well be the winner."

This may be true. However, by expanding its offerings intelligently -- not taking in excess inventory, not spending too much to expand, and creating a stronger bond with users -- Amazon may avoid a head-to-head clash with eBay, even as it co-opts eBay's strongest virtues.

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