I buy and.sold on eBay. Its a lot safer to buy on eBay these days as eBay has its protection guarantee. Issue I can see is if you purchase a GPU from eBay, you won't have warranty beyond the whole eBay protection plan. So if in 3 months the GPU stops working, you may be hooped unless the seller is honest and good. You never know, you may end up with a good seller. I did multiple of times whom were happy to help me with RMA on an PS Vita.
Anyway, as said, make sure the seller has good feedback.
Otherwise though, prices are so absymal on eBay, you may be better off like I said with a pre built. I know I'm better off. Dell RTX 3080 sells for $2,650 CAD here I'm Canada on both eBay and local listings. I got that and a whole system for $2,900.