Beware the internet! I have sold a few things online, and while I take good care of my stuff there is no way on earth I am providing any kind of warranty for anything I sell unless it is a system sold to a client. When on a used market then the buyer gets whatever the buyer gets. No way I am selling a part to someone only for them to break it and blame me for it!
That being said, it would not hurt to ask for some warranty or guarantee... but if it doesn't work there is not likely much you can do as the buyer other than a bad review on their ebay/amazon page. You would still be out some money.
25-30% off of a relatively new card is not a bad price though if the seller is legit.