Alex Tabarrok has an op-ed in the Wall Street Journal discussing how various countries regulate the exchange &/or sale of human organs. Unfortunately, in the US the quantity supplied of organs is insufficient to meet the quantity demanded, and thousand of folks die each year waiting for a viable organ.
We talked about this a bit in Econ 300 this week, specifically discussing how Becker and Elias derive an estimated market-clearing price if organ trade were liberalized in the US. This is a really nice piece that really illustrates how economists think about what supply and demand curves represent.
I also highly recommend an accompanying article in the Journal of Economic Perspectives by Alvin Roth about how repugnance can constrain market exchange.