You have no logical reason to ban them, using Auto-Buy URLs sites is the most common way to scam, but it's up to you. you must inform the seller to provide trusted Auto-Buy URLs with trusted escrow or use Bitcointalk trusted member escrow "If s/he refuses, open a scam Accusations topic against that user."

Prevention is better than cure, if there are any doubts about the seller, do not let the high discount rate scam you by risking purchase from the user.