OMC I find the examples you give interesting because they highlight to me the problems with consent that are encountered when money enters the equation - basically you end up with a load of desperately poor people who feel it's their only option. There are striking parallels with prostitution.
From The Guardian - Kidneys for sale: poor Iranians compete to sell their organs ...
In order to advertise her kidney, Marzieh has written her blood type and her phone number on pieces of paper and has posted them along the street close to several of Tehran's major hospitals, home to the country's major kidney transplant centres.
Others have done the same. Some have written in big letters or in bright colours to attract attention; some have sprayed their information on the walls of public or even private properties.
"Kidney for sale," reads one ad, carrying the donor's blood type, O+, and a mobile number, with a note emphasising "urgent", insinuating that the donor is prepared to consider discounts.
From this rather good blog article by a living kidney donor ...
If one is only concerned about would-be recipients, or one benefits from organ transplants, then a kidney market is something to consider. But if one believes that all people deserve equal consideration, then harvesting a major organ from a living person in return for money or other incentives is not acceptable. Pragmatically, there is simply no way to control for abuses and manipulations.
As JustAHolyFool points out, people who have real choice don't tend to sell sex, just as they don't tend to sell their kidneys.