A day ago, German tech site C't wrote a cautionary tale about this problem. The site warns a large number of (usually Chinese) vendors are selling fake video cards. For example, there's a large volume of "GeForce GTX 1060" cards that are sold for just 54-80EUR, whereas second-hand cards usually sell for at least 150EUR.
The site decided to buy eight cards to see what they'd get. After inspection, the cheap "GeForce GTX 1060" cards turned out to be relabeled NVIDIA GeForce GTS 450 GPUs. Basically, eight-year old garbage.
The next step was to contact the vendors. The reaction from the sellers was broadly similar, they denied the forgeries and demanded evidence in the form of photos and screenshots. Some first (incorrectly) claimed the cards were not compatible with Windows 10, while others demanded a return to China before they'd issue a refund.
Getting in touch with eBay didn't provide much solace either. Despite eBay's Buyer Protection, getting a refund for the fake video cards turned out to be very hard. Just one of the eight refunds was handled in a somewhat satisfactory way, and PayPal and eBay both failed to meet their buyer protection commitments.
Furthermore, C't reports the counterfeiters are still active on eBay. As far as the site can see, eBay has not sanctioned them, and is not proactively banning fake video card from its marketplace.