If you can find a 4070 for a good used price, I would go for a 4070. But the RX 6800 XT is a *slightly* slower in raster (3 - 5 percent I believe) and is much slower in RT, however it has more VRAM. I would pick the RX 6800XT if you need the VRAM, but 12GB should be more than enough for 1080p RT. You aren't gonna PT with either of these cards, guaranteed.
RT, DLSS = 4070
VRAM intensive task = 6800XT
Don't narrow your options to just that though, keep in mind that theres also some other GPU's out there which are good deals used. 3080's are still pretty alright for 1080P RT, 1440p non RT, RX 6800's are decent value with a alright OC potential. Etc.
If this is a new build I would recommend checking the wattage too, the 4070 requires less beefy of a power supply. (4070 = 200W vs 6800XT = 300W, though the 6800XT w/ a UV will help with performance and efficiency.) That might decide your choice too.
There are used products out there which work fine, some just don't have a warranty, open box, etc. Minor physical damage. Bought a 1050Ti pretty far back and that works perfectly fine. Same with a 1060 3GB, 2080S, a few old intel cpu's.. etc, etc.
We're not in the days were its easy to be deceived anymore; most cards that have damage severe enough to need repairs for example are usually clearly marked or are obvious in the pictures. Most sites you can buy used cards from also have buyer protection (Jawa, Ebay, etc).
It's a personal choice whether your comfortable buying used products or not. I can respect the choice not too, but they are not 'ticking time bombs'.