I could just as easily propose contracts for sales of kidneys from living donors. You have 2 of them, after-all, and if you happen to be mentally incompetent enough to sell one of them for a few bucks, who the heck does the guvment think they are interfering, eh?
EDIT: See http://slatestarcodex.com/2013/08/25/fake-consensualism/ for some more background.
"I think part of the reason is we worry that there could be at lack of judgment there, perhaps. Someone under that unattractive choice. It's related to Mike Munger's concept of euvoluntary. It's a person who is clearly under duress no matter what: even if they enter into it freely, it's such an unattractive option, to call it a free choice seems somehow perverse in itself."
http://www.econtalk.org/archives/2016/08/leo_katz_on_why.htm...
They describe setting up a consensual "kidney club". The idea being that if you're in the club and need a kidney, one member of the club is chosen at random and compelled to give you a kidney, but likewise if you have kidney failure and would otherwise die you get a kidney from a random member.
This sidesteps the usual concerns about implementing a marketplace for organs. I.e. even if you're a billionaire the only way to insure yourself against kidney failure is to enter the pool of potentially mandatory donors, and you can of course stay outside the "kidney club" and not have to donate to anyone, but then you also die if you have kidney failure.
Katz points out that the reason this doesn't work is that current law can't compel you to donate your organs, even if you've previously signed a contract to that effect, but that this creates a market failure & tragedy of the commons.
Implementing a system like this seems like a no-brainer and a benefit for everyone involved, but it's blocked by current contract law & the inability to force people to undergo medical procedures they consented to in the past.