Best place to buy a used iPhone?

Hi, I’ve bought two second hand phones lately and had a bit of a nightmare - both have/had serious problems, and having spent £110 to fix them yesterday it’s introduced new problems!

I’m not even overly fussy - I’m used to poor battery life so don’t mind bad battery health, and don’t mind a few scratches etc, but I do want an original OLED screen (both my X & XS now have LCD screens) and do want the phone to actually work like it’s supposed to.

I’m guessing I should be on the lookout for something with a warranty, but are there places where I can buy from and if it has bad third party parts I can return them with no hassle? Thank you