* if the phone is already setup, go to settings and see Find My Iphone is ON/OFF
* IF the phone is logged into previous user's Icloud?
* R-Sim or not?
* Fingerprint ID working or not?
* Battery health if its less than 80% then you'll have to replace it soon
* Check the sim tray because I have seen that most have bent/crooked sim trays, (user trying to pry it open) if it's bent even slightly.