Just buy from a site that has money back guarantees.
Ebay has a 30-day one, CeX in the UK gives a 24-month warranty on most used GPUs (and that's one of the reasons they're not as cheap as private sellers can be). I've never used them but Jawa.gg are a similar thing in the US, I believe.
Realistically, few sellers are going to jump through the hoops of recording video with serial numbers, allowing a stranger to remotely get onto their PC and poke about, or provide comprehensive benchmark runs and stress testing. If something is legit it will be listed on a site or forum where you can see that seller's reputation/feedback/past sales and it will be photographed clearly at least from 2 or 3 angles.
If something sounds too good to be true, it most likely is too good to be true - so you're gambling. The golder rule of gambling is NEVER GAMBLE WHAT YOU CAN'T AFFORD TO LOSE.
If you don't want to gamble, pay a bit more from a site that offers escrow or buyer protections.
Remember, even if you buy a card used and it works fine for a while, long enough to get through the escrow period or 30 day protection from ebay, you still don't have the manufacturer's warranty as basically no manufacturer of GPUs will warranty the GPU once the original buyer has sold it onwards.
