Things to check before buying a second-hand iPhone.

First, check the box contents, it should have earphones, an adapter and a USB cable

 Check for the original invoice, and crosscheck the phone’s IMEI number with the one printed on the box.

 Check the phone’s screen, display, and microphone.

Remove the water tray, to check for any water damage.

 Verify charging port, buttons or touch ID is working or not.

Check the camera unit for scratches and workability

Ensure that the phone has the original battery