if you are worrying about whether its unlocked and not other issues like icloud activation and whatnot, you can ask the seller for the IMEI, then check the iMEI at iphoneimei.info
If the seller does not want to provide it, explain to him you need it to actually verify if it is 100% unlock and not anything else, even if the listing says unlocked.
Dont trust a seller's word for it! I've had instances of that happening, where the listing was stated as unlocked, but when I verified it via the IMEI, it was still locked and in contract.
If the seller is adamant about not providing it, move on to the next seller. Chances are that most sellers will provide the IMEI.
If you really want to increase your chance of winning and getting the cheapest value, find a listing that ends when people in the US are normally asleep 🙂