Inspect the display and cameras
Run a full touch and pixel check, then open both cameras and test focus at near and far distances. Check the camera glass for scratches and look for dark spots, haze or stabilization problems in the preview.
Check battery health and charging
Open Settings > Battery > Battery Health & Charging and review maximum capacity and important warnings. Test the Lightning or USB-C port with a known-good cable and make sure the phone does not disconnect while charging.
Test Face ID, buttons and audio
Set up Face ID only with the owner's permission, test volume and side buttons, try the mute or action switch, and check the earpiece, loudspeaker and microphone during a call or recording.
Verify Activation Lock is removed
The seller should erase the iPhone in front of you so it reaches the Hello screen without requesting their Apple Account. Never accept an iPhone that still has someone else's Activation Lock enabled.