Well in the UK we have a shop called CEX and if I was to trade in my old 2080 I could get 179 for cash or 217 for a voucher to spend at their store, I have always traded my old parts for something new but used. My 2080 does ok in some games , but starts to drop when I play some modern games now.
Yeah I am in the UK and I know CEX. I think they do some form of warranty on their cards as well. You would probably be getting about 200 - 230 max for your card on ebay from looking at things. Before sellers fees are taken off.
I doubt they would have something like the 4070 in yet due to it being so new but they may well have the 6950x or a higher tier 30 series card.
Personally I always try and buy new so I would get the cash and buy online but that choice is yours. If you have the initial £700 + the 180 from selling your card then you could look at something like the 7900xt for about £750 or the 4070ti for around £770 - 800.
I am currently on a 2070 super at 1440p so I am currently looking at a similar upgrade
Edit : I am mostly looking at the standard 4070 over the 4070 ti though as the value to performance feels better personally