Dial *#06# to get the IMEI number. Verify it on IMEI.info or Apple's check coverage page to ensure it isn't blacklisted or stolen.
Before paying, verify these non-negotiable software and security statuses: how to buy used iphone 7
If buying in person, ask the seller to "Erase All Content and Settings" in front of you. If it asks for an Apple ID password they don't have, walk away. 2. Hardware Inspection Checklist Dial *#06# to get the IMEI number