Well, without regulation it won't necessarily be the _FUTURE_ interest in selling an organ. As I said, desperation can make a person do crazy things, and there's more to consent whether there's a contract involved and no guns pointed to anyone's head.
And while it creates an incentive to donate organs, it also allows wealthier people to hoard them. Remember that the circumstances which allow the organs of a person to be successfully extracted for transplant are pretty rare[1], they essentially have to die in a hospital of brain death with their organs intact, so even if more people are willing and legally bound, offer may still fail to satisfy demand, only this time it will be mostly a matter of who has more money. And that's not exactly fair.
By the way, as a separate issue; if people were allowed to bid for organs. And assuming you're right that rich people horded them, then wouldn't the supply rise up to meet that new demand? Wouldn't more people sign up to be organ donors? Wouldn't we reach equilibrium where the supply side matched the demand side?
If poor people were getting fabulously wealth from committing to donate organs, and more lives were saved, wouldn't we eventually reach equilibrium? The 1% only need so many organs....
Poor people with money in their pockets and less organ shortage wouldn't be a horrible scenario. Maybe it would force the real hard work of cloning organs and improving a unsatisfactory system.
I must say, if it became an outright bidding system some people who society might say are less deserving than someone else might benefit. But that happens already! Look at Mickey Mantle who received a liver after a lifetime of alcoholism (and died soon thereafter).
Or Steve Jobs who shopped around for the best region to receive a new organ (he's not FROM Tennessee after all...).
So please tell me, how would a MORE market based system be significantly worse? It seems like we have the worst of everything right now in our pursuit of the idealistic fantasy that the system treats everyone equally and that organ donation should be a purely altruistic gesture.
And while it creates an incentive to donate organs, it also allows wealthier people to hoard them. Remember that the circumstances which allow the organs of a person to be successfully extracted for transplant are pretty rare[1], they essentially have to die in a hospital of brain death with their organs intact, so even if more people are willing and legally bound, offer may still fail to satisfy demand, only this time it will be mostly a matter of who has more money. And that's not exactly fair.
1. http://www.organtransplants.org/understanding/death/