I agree that a listing fee hurts poor players - its not tough to envision that a up-front fee could actually prevent someone who's broke from selling something.

What about, instead of a listing fee, a much more painful fee on price adjustment? Say, 10% of the new charge every time you chance the price on an item you have up, combined with a cool down that would lock you out from selling something, for say 1 hour, if you just took that item off your board.