There are many reasons why you would buy a used graphics card. First and foremost, the savings, of course. With the mining madness now over, we've been given the opportunity of buying graphics cards on the cheap. Also this past generation of GPUs also received a much longer than expected life cycle, meaning you are not getting overly old GPUs, but fairly serviceable gaming hardware.
Slow computer got you down? Maybe a hardware upgrade should be in your cards, if so this guide to the Fundamentals of Upgrading a PC is an invaluable read. PCSTATS Tips