I use Amazon a lot and I've never received a single incorrect or previously used item, and every return has been accepted without problem.

(They're not perfect -- I've had leaking shampoo bottles, occasional delayed shipments, broken mirrors, and broken packaging leading to missing parts, but at the same rate as any other retailer.)

So Amazon definitely wins over eBay here. But sometimes Amazon is out of stock but there's a listing on eBay, so...