In The Ethicist column in the New York Times Magazine last month, Randy Cohen talked about organ transplant sales being unethical:

For a system of acquiring organs to be ethical, it must be equitable, which is not the case when one economic class is exploited (and put at significant medical risk) for the benefit of another. And exploitation it is when the seller is not making a truly voluntary decision but responding to financial desperation.

Is it unethical to hire a maid who is financially desperate? If I had trouble getting a job out of college, I would be financially desperate, but I would be very grateful for the opportunity to sell my labor.

Organs are different than jobs. But the difference is not financial desperation.