This is just out. I like the point Professor Taylor makes near the end about how virtually everyone involved in the transplanting of a kidney from one person to another is paid except the person who gives up a precious part of his/her body. (Of course, the recipient is not paid but that's for obvious reasons: that person benefits big time.) Not only that, but some of the people--I have in mind the doctors--are often paid very well. The restriction on sale is, in effect, a price control of zero.
By the way, I'm an advisor to something called LifeSharers. It's a creative response to the restriction on selling kidneys and other body parts. The idea is to give people an incentive to give up their body parts on death by having them qualify, by joining, to get other people's body parts if they need them in the future.
Signing up for organ donation is a social contract. In the case of organ scarcity it is appropriate to favor fellow organ donors over free riders. When it is time to allocate a scarce resource, it is fair to assign priority to people who are willing to both give and receive.
Notice that, unlike all those other phony social contracts (how do I not remember signing?), this really is a social contract.