Should I buy a used 980 ($280) or a new 1060 3GB (+$325, used ones are hard to come by) or even a new 1050 ti ($240)? 6GB is not an option since it costs +$400 and used ones are nonexistent.
Buying used graphics cards can be an issue... you don't know what that gtx 980 have been through... I would say new 1060 3gb, so you can also have warranty, and the difference is not that much.