Like I said, it depends on how risk averse you are. Financially, there was no way for me to spend to my budget, and upgrade my graphics card, without going 2nd hand.
We also need to consider what you're upgrading from. My old card would have had only a year warranty left, and my 2nd hand 1070ti has 10 months manufacturer (MSI honour cards without purchase receipts that I've seen, based on manufacturing date). Or I'd have 3 months from OCUK. So I've missed out on 2-9 months warranty, but got 40% boost in performance. If it dies in 2 years, then it's no worse than my original 580 dying at the same point, neither would be within warranty anyway.