EL CAJON, Calif. -- Almost anything can be found for sale on Craigslist.com. If you've been on the Web site recently, you might have seen a California man's ad offering up a kidney for $100,000.

In his advertisement, William Miller of El Cajon, Calif., wrote: "Kidney's yours. It's As simple as that. Do it as soon as you want it done."

Miller and his fiancée are unemployed and he thought selling an organ would be a way to pay the bills.

"If I got a hundred thousand dollars plus medical expenses, I could put money aside for the kids to go to college. I could pay for myself to go back through college to get a better job," he said.

In the five days Miller's posting was up, he received two messages: one criticizing his ad; the other asking for details.

Miller's not alone. An increasing number of people are looking for new ways to raise money by selling, hair, plasma, eggs and sperm. But selling organs or tissue is illegal.

Lisa Stocks of LifeShare said the line is crossed "the minute you start talking monetary reimbursement, giving money to someone, giving anything to someone, anything that's called 'valuable consideration.' You can't give a car to someone for a transplant. You can't give anything to get a transplant."

San Diego State University ethics professor J. Angelo Corlett said it's a highly charged ethical debate.

"In some cases it might be justified on moral grounds," Corlett said. "But we can all imagine many cases where it is about greed, it is about exploitation. And I think most of us would agree those kinds of cases aren't justified."

Regardless of the arguments, U.S. federal law prohibits selling human organs and tissues. Any compensation beyond medical expenses can mean fines and jail time.

The Craigslist posting has since been taken down. The Millers said they had no idea it was illegal.