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...

Yeah, buying anything 3rd-party through Amazon is basically like gambling, and I don't recommend it. Ebay is MUCH better and safer than Amazon 3rd-party: you can look at seller feedback easily and see if a seller is a trustworthy, long-time seller or not. Amazon is only good if you're buying from Amazon itself.

My "new" (and I don't claim this is unique at all) rule for Amazon is that I no longer buy 1) anything that goes in or on my body, 2) is a safety device (carabiners, rope, flashlights, etc.), 3) is a fire hazard (charging bricks, etc.), or 4) high counterfeit risk/low barrier (SD cards, etc., as I do wedding photography professionally).

And that's even from the "Manufacturer Store", "from Amazon". Still not worth the risk of co-mingling (which I'm skeptical even happens, even if Amazon says a seller can opt out of co-mingling).