Eugene Volokh has posted the introduction to a new article that he has coming out in a few months. Essentially, he seems to be arguing that, based on other areas where people's lives are in danger and they are allowed to take extraordinary measures, even when they are harmful to others, the same should be allowed for organ purchases.
I, of course, would not have relied on the Roe and Casey abortion cases as I believe those cases were wrongly decided and should be reversed. Still, I find the argument interesting.
This of course ignores the policy arguments for banning organ sales, namely that the poor and desperate will be taken advantage. I presume in the actual article he'll address that to some extent.