I've never used StubHub. When you buy tickets, does a seller profile appear next to the listing, as in Amazon? If so, a cheap move by this seller should at least put a bad mark on the seller's public profile. That could benefit reputable sellers, though. Of course, anyone selling their first tickets on StubHub would be at a disadvantage because they'd have no reputation.
They can't exactly force that either, but they should be able to force penalties or restitution if they fail to perform.