In a less than ideal world...

... it's really simple: what if a poor man has no good options, and decides to sell his kidney to a millionaire who wants to save his own son? In a voluntary society, the millionaire's son's life would be saved, and the poor man would have some money to feed his family. In a coercive, Statist society, the arrangement would be outlawed on the grounds of exploitation, the millionaire's son would die, and the poor man would have less to provide for his family. I'd rather live in a society in which the poor had more freedoms and options, thank you very much -- even if a rich kid survived it all!

Then again, my ethics may be skewed.