Can. ti used graphics cards worth enough to buy. 1080ti theoretically and 2080 performance close to, slightly stronger than the 2070super, significantly stronger than the 2070, slightly weaker than the 2080super, significantly weaker than the 2080ti. However, 1080ti's 11g of video memory is 2070s, 2080s, 2080s can not be compared. Used 1080ti's are available for under 3500. But consider the following:
1, to ensure that the graphics card is good, most of the second-hand graphics cards are repaired and sold again. So make sure you test the card to see if it's good.
2, the length of the warranty, even if it is a used graphics card, there is a warranty time, although the warranty is not long.
3, the price should be very cheap, if the price is expensive then just buy a new graphics card.