I think people should be allowed to sell one of their kidneys if they want to do it but only on their own free will.  That could pose a problem.  How can you know for certain that a person is not being coerced/threatened, etc into giving up one of their kidneys?  I cannot imagine needing money so badly that I would consider selling one of my kidneys; however, if one of my loved ones needed a kidney, I would not hesitate to give them one -- no cost.