I have bought all my round 90ish phones in my collection second hand otherwise I would have no shot at affording it. I have bought a couple of lemons yes, but overall I got mostly quality stuff
In my opinion if you set a limit on how much you want to spend and keep looking around that price point you can get some amazing deals. My top 3 are a Galaxy Note 9 in almost mint condition for 70€, a LG G8S for 60€ in excellent condition and a Galaxy S10E for 50€ with a cracked back (easy fix btw).
Also the most important things to look out for are screen damages. OLED burn-in still is a very common issue and many LCD screens also have problems with very uneven backlights. I always demand a totally black and totally white image of the screen beforehand for that reason.
Also with battery life its less of an issue than one might think. I only got a handful of phones where the battery life was really unusable (less than 3h of SOT). I remember the HTC One M8, 10 and U12 Plus as well as a Sony Xperia Z5 Compact, Honor V8 and Nexus 6P really being in an unusable state when getting them in terms of battery. Not bad i.m.o. for a sample size of 90.