Yes, the word 'might' is important which is why I chose it. I don't say that it definitely will emerge. But the surrogate mother case provides a good example, I think. As I guy, I'm running a risk commenting on this because obviously I don't speak from experience, but it seems like there is a fair market price in this case. Surrogate mothers arguably make more than they would in a year at a median-paying job. It doesn't prima facie look like a case of exploitation, but I'm open to being corrected.
Speaking of side-stepping, you kind of talked around the concrete argument and examples I gave. You talk a lot about the generalities of markets and slippery slope-type arguments about where such thinking might lead, but you don't respond in any detail, as far as I can tell, to this specific case or how legalizing the sale of kidneys -- with the appropriate regulatory oversight of course -- leads to the deliterious consquences you envision.
I agree with you, however, that economics is not morally value-neutral. I stated up front that we have intuitions about what it's not okay to do for money. My question is simply why does selling a kidney fall under this heading whereas other examples of 'selling one's body' arguably do not? Your answer seems to be that introducing market forces will make the exchange value-neutral. But isn't that just conceding the point to your capitalist interlocutors? Why not talk about the ethics of markets across the board? The kidney case, then, becomes just one more example, albeit a difficult one, of how we assign value, both moral and economic.

Grue, thanks for the link. I may have listened to that episode in the past, but I'll refresh my memory when I get the chance.
With respect to Richard's comment, it probably warrants more by way of response than I gave it the first time around. In retrospect, my response seems a bit terse. I was writing in a hurry, but I wouldn't want that to be mistaken for dismissiveness on my part. 
Obviously, I haven't worked out the logistics of my tentative proposal in any detail. That wasn't the purpose of the post. I just wanted to stir the pot. If sale 'to the highest bidder' were the only option, of course, the proposal would be problematic. But sale to the hightest bidder -- which I agree is the least desirable arrangement and most likely to be abused -- is not the only way the sale and distribution of kidneys could be organized. What about a gov't organ bank that set the market price and incentivized the sale only in cases where scarcity demanded it? Again, I'm just throwing out some ideas.
I'm also sensitive to the concern about viewing persons as commodities. Most morally sensitive people share the intuition that it's wrong to view persons as mere commodities or means to an end. But I'm not entirely sure how this principle applies here. Is the act of 'selling one's body' itself somehow demeaning of the person? Is this so in all cases? In this case specifically? If the person does so autonomously is it still so demeaning that the state has an overriding interest in preventing it? These are the questions in the neighborhood of this issue. It becomes an exercise in what, to my mind, looks like arbitrary line-drawing. Are the general principles we rely on elsewhere useful in these cases?
Also, are there cases in which compensation can actually affirm personhood? Whether we like it or not, the way our society assigns value is through money. Perhaps I'm sensitive to this because over the past five years I gave a lot of myself to a job for which I wasn't fairly compensated. This has a profound effect on one's sense of value and can rightly be described (I think) as a denigration of one's personhood. I don't know if this principle applies here, but there's a fine line between volunteerism that's noble and volunteerism that exploitative. I can envision scenarios in which 'giving out of the goodness of one's heart' or out of a perceived sense of obligation, are more exploitative than setting up mechanisms of fair compensation. Again, whether this applies in the kidney case is debatable.
P.S. I hope that, ultimately, this will be a moot issue, as medicine advances and we can clone replacement parts in the lab. But, still, the process would likely be expensive and may still cost the recepient some money. Would this be less morally objectionable, however?
 

I suppose I'm curious about what makes the surrogacy case dissimilar to the kidney case for you? Why is a woman's womb a legitimate domain for markets and not someone's kidney?
I think we might also be in danger of talking about "something called 'the market'" in too abstract a way. Markets can be very complicated entities, but the basis for markets is relatively simple: property rights plus contracts. We generally think this is fine when the arrangement is voluntary and we generally acknowledge that individuals have a high degree of sovereignty over their own bodies. Again, people already sell blood, sperm, eggs, wombs, etc. This at least sets the stage for extending markets to cover kidneys.
The only real issue that I can see -- and I think that this *is* central to both Sandel's and Satz' position -- is the problem of weak agency. In other words, are the persons giving up their kidneys doing so in an informed, voluntary way and not being coerced by some external agency or circumstance. I acknowledged in the original post that these are practical worries. But there are at least two responses available: 1) these worries may be overcome by regulatory oversight (we shouldn't generalize from black markets to regulated markets); 2) the problem of weak agency applies also to donation (as opposed to sale).
But these are all arguments on the individual autonomy side. You rightly bring up the issue of our collective concerns. Well, how about the argument that allowing a market in kidneys would increase supply and reduce the suffering and death of 75,000 Americans on waiting lists? This is a social welfare argument and I think it ought to give us pause. We can talk idealistically about some things being too precious to be reduced to market forces, but in the meantime there are a lot of concrete cases of suffering that go unanswered. It's an exercise in utilitarian calculus, I know, but it speaks to the 'greater good' aspect of the argument.
In any case, it's good to bear in mind the bigger picture, because our moral intuitions may lead us astray. The idea of using someone, even if they are fairly compensated, for spare parts rightly causes a twinge of conscience. However, we typically don't feel that same twinge of conscience in the case of the tens of thousands per year who die awaiting a transplant. This is the problem of silent suffering and it looks suspicously like a cognitive bias in our moral reasoning.
At the end of the day, I suspect that one's position on this issue comes down to how much weight they place on individual autonomy and how much weight they place on the aggregate suffering of those on wait lists. Clearly, kidney markets aren't ideal and aren't without their problematic moral consequences, but we're seldom given the luxury of choosing between ideals. The pragmatic question, then, is "are kidney markets better (or at least no worse) than the alternative?" Tentatively, I still find myself saying 'yes.'