^^ His terms are fair. I had a very close shave with my own G80 when the buyer reported back that it wasn't running on his system and he was using some pathetic local PSU to run it. If he had persisted and destroyed my card, he could have reported that it was defective. I was very firm and stood my ground, offering to refund the cash immediately but that he had to finish the return the same day and remove it immediately from the system.
All you have to do is go to his place, and satisfy yourself that it's working. How difficult is that? Put it under any stress test, and the only reason to suspect anything is if he does not not let you run a particular benchmark or game. If it doesn't work with your equipment after that, it's an issue with your system.
If you're not comfortable with these terms, don't buy. You can't force him to accept your terms, and he can't force you to buy on his - it's always mutual, there's no book in this market.
I'm reporting both these posts to mods for deletion (yours and mine) as they are sufficiently OT.