Even if the answer is "nothing" it still makes sense they're selling on ebay—people don't know that they're buying a brick, and I'd bet that's extremely difficult to contest via ebay/paypal.
According to the link above, people know they are buying bricks.
I'm sure there are some folks trying to pass off locked iPhones as legit, but there are also people selling them "for parts" on Craigslist and I'm sure a load of other sites. I was just shopping for some test iPhones for work and there was one dude on Craigslist with a dozen or so new iPhones, all locked for like $80 "for parts".