But last week a graduate student at George Mason University advertised a kidney for sale on Yahoo! Inc.'s auction site. The posting, which the student said in an e-mail was an "end of the semester" prank, offered the organ for $5,000,000. It received only two bids, both from the seller's friends. Yahoo! officials called the incident highly unusual. "It's definitely against our policy," said Susan Carls, senior producer of auctions at Yahoo!. The company would not confirm that the item actually appeared on the site, but the number sequence appearing in the posting's URL corresponded with the numbers assigned to other items sold during the week. It would not have been difficult to post an organ on an online auction site. Users of Yahoo! -- and other auctioneers such as eBay -- sign an agreement before posting items, saying they will not sell anything illegal. But the promise doesn't stop people from posting whatever they like. The sites allow users to hawk almost anything, monitoring the items only after they reach the site. Selling organs is illegal under a federal law passed in 1984, and many organ-donor organisations blame the shortage of available organs on transplant pranks and myths. "If that happened on our site, we'd definitely take it down," said Brad Handler, director of policy at eBay. Yahoo! has workers monitoring the auction sites 24 hours a day, seven days a week, looking for illegal items, Carls said. "We're on the hunt for obvious things like drugs and guns," she said. When the monitors come across contraband for sale, they remove it. Carls said the company has had to pull items such as live pets and antique guns from the site -- which, although not necessarily illegal, are against Yahoo!'s policy. The monitors keep a particularly close eye on the section of the site labelled "others," where suspect items often appear. Most such posting come from teen-age jokesters, Carls said. "But I am waiting for the day when something serious come up," she added. In addition to the monitors, auction site visitors also blow the whistle on posters of potentially illegal items. The auction community is a very active one, and devotees have developed Web sites such as AuctionWatch to ferret out fraud. When Yahoo! discovers an illegal item for sale, it removes the object, then bans the seller permanently from the site. It does not contact the seller. The company won't turn over information about its customers to law enforcement, citing privacy policies. Privacy is a sensitive topic among many online companies. Last year, Internet services company America Online Inc. came under fire for releasing personal information about a gay sailor to the Navy.