BBC's Simon Clarke: "The incident raises the wider issues of less scrupulous operators"

The macabre bidding started at $25,000 but had reached nearly six million dollars before the California-based company stepped in.

The seriousness of the bids made for the kidney is not known, but existing organ trafficking from places such as India can provide illegal organs at much lower cost.

Fake offer

The offer is almost certainly false - many strange or impossible items appear as lots on eBay - but selling your own organs also forbidden by US law and punishable by up to five years in prison or a $50,000 fine.

"Obviously these are pranks," said eBay spokeswoman Kristin Seuell. "But we don't want to leave anything to chance."

The seller, identified as "hchero" from Sunrise, Florida, advertised a "fully functional kidney for sale."

The description read: "You can choose either kidney. Buyer pays all transplant and medical costs. Of course only one for sale, as I need the other one to live. Serious bids only."

User alert

Kevin Pursglove, an eBay spokesman, said the company lists 2.5 million items each day and has no system to spot questionable offers unless alerted by users.

"Any time you have an open trading environment with almost six million registered users, you're likely to see somebody who tries to bend the rules or pull a prank," Mr Pursglove said.

The company also claims that this sort of sale is often attempted in all sorts of marketplaces but is actually easier to stop on the Internet.

However, Dr Thomas Reardon, president of the American Medical Association, was surprised by the supposed sale. "I'd question the ethics of any doctor who would participate in that kind of transaction," he said.

Organ shortage

An estimated 44,000 US citizens are waiting for kidney transplants.

"The need for organ donors is obviously great," said Barbara Hofstein, director of the National Kidney Foundation of Northern California. "Attempting to sell organs opens up an enormous range of potential abuse."