This debate is evolving quite nicely. Not only is it turning out to be good clean fun, but I think we've also come down to the nub of our differences: On the one hand, I think people (myself included) are crazy, so the last thing we should do is let them do something as self-defeating, dangerous, and irrevocable as selling their organs. On the other hand, you think people are sensible, and since some people are dying for want of organs, the last thing we should do is keep other folks from selling some to them.
You argue against me in two ways. Specifically, you ask, if people are such loons, how can I allow them even to donate organs? Since I accept living organ donations, I must permit organ sales because nothing makes them different except the involvement of cash. More broadly, you argue, if people make decisions as nutty as I say they do, then we could not let them make any decisions at all. It "would shut down every employment and financial market in the entire world."
On the first point, don't assume I am so enamored of living organ donations. I think we should worry about them, too. Studies have shown that people often decide to donate a living organ instantaneously, before the risks are understood. It's telling that a 1997 study found that the higher their education, the more ambivalent people were. Furthermore, as a study in the April 15 Transplantation shows, many donors report that they did not feel wholly free to say no. The problem appears to be that families, society, and even hospital staffs look down on people who say no as being selfish and cowardly. No matter what we do, poor decision-making and social pressure will play a role in living organ donation decisions.
I accept such donations, however, because the other thing studies show is that donors rarely regret their decision. A recent People magazine (another vital information source for me) tells the story of a man who gave his brother one kidney only to have his remaining kidney go into failure years later. The man is now on the transplant waiting list. Yet even he did not regret donating his kidney, for it has meant that he has had his brother all these years. The lack of regret is a sign that for most people donating an organ to save a sibling or a child is the right decision.
The fact that money is involved in organ-selling makes a world of difference. The organ donor bets his life for the sake of saving a relative. But an organ seller would be betting his life for the sake of money. And like a gambler who bets his house, the organ seller would generally be making a bad decision.
This is, as you say, a difference in opinion about the "background probabilities." Both of us agree with laws that prevent people from choosing to donate their heart while alive. Why? Because we both believe that the background probabilities are that a healthy person who agrees to be killed is being irrational, coerced, or exploited. I do not think it is a huge step to think that someone who sells his organs is in a similar position.
As for your second argument, accepting the overwhelming evidence that people are often irrational decision-makers doesn't mean we have to shut down markets. We let people make all kinds of crazy consumer decisions. Witness the entire body piercing industry. (I've noticed the latest thing among Boston's hip is getting one's ears pierced with a post the size of a quarter. It makes their ear lobes look like straphanger handles in subway cars.) We allow people to make dumb choices for two reasons. First, because the alternative is tyranny. And second, because part of the purpose of human life is charting your own course. We draw the line, however, at decisions that are not only most likely irrational but also have terrible consequences. Like selling one's organs. This kind of limited paternalism is not tyranny. It is part of a good society.