Yeah - I've had my eye on a few things there recently. The general rule of thumb is to avoid these listings though, since people will pay a big premium on these listings. You can pick up NFL auction listings on eBay for a fraction of their sale price. I have bought 2 game used Bucs jerseys which were originally on NFL auctions (with certification / PSA/DNA) and paid quite a bit less than the original purchasers did.
I daresay the odd thing slips through for a decent price, but generally speaking, you'd pay more for the jersey than you would elsewhere.