Is this a local buy or online buy? If it's local, do you already have a system installed? You could give ask if you could give it a test run or maybe even ask the seller to test it in his setup if he is able. If the seller gets defensive and up in arms about testing it with you there I would just walk away unless you're willing to low ball and repair it. If it's online that's a tough call.
BTW I got all sorts of mixed up when I first read your post due to having the same avatar. I was thinking to myself I don't remember starting this thread....