Friendly advice: be careful.

I have brought loads of 2nd hand Android phones over the years (normally when im temporarily bored with iOS or want some emulator action) and it can be a minefield with regards to battery health.

If the phone is over a year old make sure it had at least 4000mah at launch otherwise you'll be exchanging it in a day. Sometimes you get lucky (my old P20 Pro still goes for a full day) but this is rare.

To be fair iPhone has the exact same problem, its just getting a new battery fitted to an older device can be done on most high st repair shops and on most phones you can see the health. Always check before buying!