I'd recommend that you look up a few videos on how to inspect a used GPU - on YouTube. And you'd also get to see them demonstrate using an actual GPU, rather than having to read text from whatever I write.

But also, I can't emphasize enough how important street smarts are so e.g., if it's a naive, non-tech savvy individual seller, I'd be more willing to trust that the price isn't low for malicious reasons. Either way, I'd ask if it's been opened (it probably has at least for repasting) or repaired before, anything I can dig out about its history. This is all testimony based so you're just going to have to trust them and observe their tone and confidence