Just as backup I would email the seller if your interested and get them to say in writing or email that they will stand behind the clean ESN or accept return and full refund in the event it doesn't pass the activation. Just one extra bit of protection on your side if it turns out to be a bad ESN, plus you also have proof over EBay's email system via seller that they would stand behind it.
Its just more security on your part, if the seller is true they won't mind providing this.. If they give you some static about it then take warning, red flag something maybe wrong here...