Money Changes Everything

In the first chapter of SuperFreakonomics, we write at some length about the economics of prostitution, both among street prostitutes and a high-end call girls.
One of the most interesting aspects of prostitution is that it involves a good or service (or whatever you want to call it) — sex — which, when undertaken for free by consenting adults is legal but which becomes illegal when money changes hands.
Can you think of other goods and services that share this trait? Let’s also consider examples where money doesn’t necessarily make the practice illegal, but at the very least taboo or socially repugnant.
I will put a few more examples below the fold, so as not to ruin the guessing game, but I am hoping you all can collectively expand this list many times over.
A few other goods and services that come to mind: human organs; children (you can put your baby up for adoption but cannot sell it); and — my favorite, suggested by a smart fellow I met recently — political favors.