I had that happen once - a friend and his wife needed to go to California at the last minute. Airfares were almost $800 each, but there were FF tickets available. He asked if he could buy two tix off me for $750, and I agreed. He paid the taxes and fees ($165 total, almost all the late booking fee), but he got his tickets for a hefty discount and I pocketed some easy money.

I wouldn't offer to sell tickets, but if someone asks a fair price, I wouldn't mind doing it every now and then (don't want to get flagged, so wouldn't do it too often).