moral limits of markets

'Anybody Need a Kidney?' or 'What Are the Moral Limits of Markets?'

I'm in the process of looking for work. In the meantime, I'm a bit low on cash. A friend of mine recently joked that I could always sell a kidney. Well, I'm not that desperate (yet), but the fact remains that I can't sell a kidney, at least not legally. This got me thinking: should I be able to do so?