Thanks for your question.

That was a real concern of mine, for a long time. Business Wise: 1. eBay has a really good buyers protection. I've had items that were not delivered or dysfunctioning, and I got a full refund. No questions asked. 2. Make sure to check the sellers rating. - pcprice.watch also includes the sellers' rating.

But I understand that you are more asking about my approach for a GPU to work hardware-wise? Technical testing for GPUs: 1. I do PCB inspection, and I check the power connectors PCIe contracts. 2. I found out GPU-Z to be useful 3. And maybe the most telling gaming benchmark suite: 3DMark or gaming sessions

So far I haven't had any incidents that had made me lose money, and I do flip PC's quite often :)