I've looked at award bookers' websites out of curiosity. Seems that even if you were interested in selling your miles, you get a REALLY raw deal: they pay on the order of a penny a mile. So there's another reason not to sell your miles to those people, in addition to the ethical reasons and the possibility of being caught by the airlines.

Depending on your tax bracket, you could probably do better than a penny a mile by donating a ticket to charity (like the one that flies cancer patients to treatment centers) and then taking a tax deduction.