Would it be better to grab a used 580 for 120 or a 580 on newegg $160-200.
For $40 I would go new. It all depends upon your budget.
If you are getting a used one for a good price then you should definitely go for them because it doesn't matter whether you use used or new because most of the used GPUs are not that much used.
I will never buy used again, even though it was probably mostly bad luck:
Seller had great reputation and bought what seemed to be a great deal in December. "Excellent like new condition." The GPU seemed to be running hotter than it should, so i opened it up and it was absolutely filthy. Worked hard on it to get it cleaned up and apply thermal paste and it ran fine until recently where it always freezes up.
Never wanted to buy used in first place, but needed to upgrade and 2080's were awfully high considering only 8gb vram.
So if you have a choice, go new.