CEX is definitely worth a look... they offer a limited warranty and they (very) tacitly inspect the GPU's before they sell them so you should know that it will at least work. The downside is they tend to be more expensive than you might get on say e-Bay. You might not get a 'great deal' but you'll get a card that does actually work and if it doesn't you'll get your money back - and you won't just get scammed like you might do on craigslist or gumtree.
The other thing you can do is look on Facebook marketplace, but if you do this you need to really vet the sellers... i.e. look for people that are local, have full profiles, are very happy for you to come and look at the card before you buy it/see it working etc. It sounds slightly harsh but if you're buying a GPU for gaming, then really the person selling it should seem like a gamer. If they don't seem like someone would be a gamer, or if they are selling large quantities electronics then it's probably either a scam or at the very least a bad deal! The 'unicorn' is finding some wealthy middle aged gamer-nerd selling his 1080 Ti because he has upgraded to a 2080 Ti so he can get a few more FPS in minecraft or CSGO... i.e. a GPU that's basically either been unused or under very light loads.
It can be quite labor intensive but you can get really lucky on FB marketplace and find cards that have been looked after/kept clean and are simply being sold because the seller is another gamer who is just upgrading.