Bought a 770 almost a year ago, as I couldn't get a 3XXX series and I wanted to play something at Christmas. The card arrived and it wasn't in good condition, fans were rattling really badly and the moment you played something or did a stress test, it would overheat and the PC would shut down itself.
I didn't want to send it back, as there weren't any more available, so I cleaned it up and changed the thermal paste, and that solved the temperature issues (not the rattling though). Used it for about six months and sold sold it when I finally got a 3070. CEX didn't buy it from me, as they said the fans weren't in good condition (and that's how they sold it to me). I'm pretty sure that GPU wasn't tested at all, you could tell it wasn't in good condition just by turning the PC on. Wouldn't buy an expensive GPU from them if I'm honest.