Professor Stephen Mathis has come across my post responding to his op-ed, and he comments, in part:

I think the ultimate problem with devaluing people or their organs is problematic precisely because it makes them vulnerable to more powerful folks. But I do disagree that disallowing a price tag on organs makes them worthless: I think it simply makes them incommensurable with money, which marks off their special status as things that are unlike everyday commodities. The same goes for laws outlawing the selling of sex. Making it impossible to buy or sell sex doesn’t make it worthless, rather it delineates it as something so special it shouldn’t be open to the pressures of the market (that usually come from the powerful/rich).

I don’t know Mr. Mathis’s background, but I’d suggest the possibility that he’s just never encountered a situation in which he’s needed a sufficient amount of money that would justify the sale of a body part. I’ll tell you the honest truth: I’d part with certain bodily properties if I could thereby erase my debt.
The economics are unavoidable: Every body part has an abstract value; that we disallow their sale just removes the motivation to assign a dollar amount to it. The same is true of sex, although the value is so much lower, and unlike organs, its sale doesn’t deprive the seller of its use, so some people will always make the transaction, whatever the law says.