well. at least he willing to take the responsible. =) maybe he didn't check it properly. there's so few seller will did what he did.
he may just refuse to refund / pay you as you checked on the spot is ok. he can argue with you that is you the one who spoil the fan.
honest speaking you also have responsible on the faulty fan. you should have notice it when you are doing COD or at the first day you plug it into PC. after couple day, u complain the fan is rosak, the seller without talking much straight away bank in some money for u, or even agreed to bank in. what you want some more?
if you want the card 100% gurantee is good, then maybe you should pay more and buy a new card from shop. cheaper means you might have to take the risk as it is second hand stuff. so it's not fair just to blame him.
anyway. glad to see this case settled.

case closed, same goes for the thread.