Recently, I saw a movie, Lilya 4-ever, the story of a 13-year Russian girl, abandoned by her mother, and cheated by her boyfriend into becoming a sex slave in Sweden. She eventually commits suicide. Informed consent is tough to establish in any trade that involves physical violation. The doctor should know better, but I suspect if physical or biological violation of any conscious life form could ever be considered as totally voluntary. Natural instinct will be to resist such violation.
Having said that, one may consent to lose some to gain more - definition of trade - but a necessary condition is complete information. The sordid stories on drug trial outsourcing - see here and here - are not very reassuring, not for a market for organ trade anytime soon. I would be particularly wary of "informed consent" in totalitarian countries such as Iran and China!