I keep telling my clients "buy used components aside from fans and HDD/SSDs". It's only been a few months that I've had a few listen to me, and every time they've been much much happier not paying tax and getting the same product at the end of the day. I also avoid buying ancient cards like the 580 or lower series. This isn't even counting into the fact that I visually inspect the cards and most look better than a card in a clients computer for 2-3 months uncleaned. The bottom line is people sell graphics cards because they upgraded or need cash, and it's pretty rare for someone to sell you a broken card unless you're blind and have no idea what you are buying.
Tips: Check bolts for paint damage (being undone is an instant red flag), check pins to see how many times it was put in, check under the heatsink and look at the TIM pads; how much dust has built up? It's hard to clean that stuff so it helps tell you how old a card really is! And last, ALWAYS bench an open box product before buying. I've NEVER bought an out of box video card that looked even slightly used unless I saw it boot something in 3d.