It doesn't matter if they don't offer that. If it actually happens that it's dead when you get it, you just take it up with paypal. They will get you your money back.
And really, nobody wants to try to sell a bad card. So the only way it would really be DoA is from shipping. Or terrible not anti-static packing, which would surprise me from anyone able to sell a part and take it out of their computer.
But yeah, if it's used then presumably it's been running a while just fine in something. Most likely past the time period of likely failure.
I know that when I sell, I don't offer any kind of return stuff. But that's because I just don't want to hear about whatever it is ever again. And I never have, other than feedback that's like 'this was an amazing deal, great seller'