Incentive vs. Coercion
Publius at Obsidian Wings outlines his argument against allowing kidney sales. It's one just about every libertarian has encountered, and been utterly bewildered by, before:
Here's an example. Let's say Wal-Mart decides to start offering people $5,000 per finger. (Let's assume scientists have found a chemical in finger bones that helps break unions). Under the liberal view, everyone is "free" to take that offer or to decline it.
But this "freedom" isn't all it's cracked up to be. From an opportunity cost perspective, Wal-Mart's offer exerts more force on someone with a salary of $25,000, than it does on someone who makes $100,000. The former is essentially throwing away 20% of their yearly salary, while the latter is throwing away 5%. Both individuals are "free" - but the lower income makes the offer harder to refuse for the $25K person. It exerts more force on that person. To say, then, that these two people are both "free" misses an important difference between them.
The problem with this argument is twofold: First, it relies on a deeply flawed conception of what coercion is. If Wal-Mart were to go around making people poor and then offering to buy their fingers for $5,000 apiece, that would be coercive. But Wal-Mart doesn't make people poor*. They'd be poor with or without Wal-Mart and its funds-for-fingers offer.
So to say that Wal-Mart's hypothetical offer exerts more "force" on the poor is true only in a very broad sense of the word "force"—i.e., one that includes force of persuasion. The poor aren't having their options limited by Wal-Mart; they're having their options limited by being poor. By offering to buy their fingers, Wal-Mart expands their options.
Which leads to the second problem with this argument: It can be accurately summarized as follows:
Premise: It's terrible that some people are so poor that they might decide that selling a kidney is their best option.
Conclusion: Therefore we must make sure that they don't have that option.
The premise is tenable, but the conclusion—that we should make the bad situation the poor are in even worse by cutting off what they consider to be their best option—is absurd. A somewhat less insane conclusion to draw from the premise is that we should find some way to raise the standard of living of the poor so that selling a kidney becomes a less attractive option**.
The fact that we have not done this yet*** is not a legitimate reason to support a ban on kidney sales. The two issues are entirely independent of one another. If the poor are to remain at their current standard of living, they're made better off by being given the option to sell their kidneys. If we do find some way to mitigate poverty further than we already have, the poor are still made better off by being given that option. And it's very unlikely that legalizing kidney sales will prevent further attempts at poverty mitigation.
Really, there's only one tenable argument against legalizing kidney sales, and that is that the poor are, by virtue of their lower intelligence, simply not competent to decide whether or not to sell a kidney. While I certainly do acknowledge that the poor are, on average, quite a bit less intelligent than the middle and upper classes, I'm just not comfortable with that level of paternalism except in truly pathological cases. And I don't think there are many leftists who want to go down the road of deciding what rights should be denied to people with low IQs. Nevertheless, I will grant that this argument is at least logically coherent.
It's worth noting that this refutation applies to all arguments that take the form of "It's coercive to offer the poor money to do X."
*Yes, I'm aware that many leftists believe that Wal-Mart does just this, but they're too far gone to bother with.
**Actually, since most people put a very high value on not dying, this would most likely just result in an increase in the market price of kidneys, and the kidneys would still come from disproportionately from the poorest among us. But I'm assuming that there's some absolute standard of living high enough that most left-wing opponents of kidney sales would drop this particular objection.
***In fact, we have a truly excellent poverty mitigation program which has increased the poor's standard of living severalfold in the last century, and several others which have increased it moderately beyond that. (I am of course referring to capitalism and to various government welfare programs, respectively.) What I mean is that we haven't mitigated poverty enough for people to stop making this argument.