(1) Verify the IMEI Number from Both the Body and Software---
It’s possible that the iPhone you’re buying has a replaced body. That means you don’t know what it’s been through. It might have been submerged in water or it could have survived a bad fall.Even though the exterior might look new and shiny, the internals could have a whole different story to tell.On the back of the iPhone you’ll see the IMEI number. Compare it with the one in the OS. Go to Settings -> General -> About, swipe down to IMEI, and compare them yourself.
(2)Activation Lock (Make Sure it’s Not Stolen)---
Ever since iOS 7 came along, Apple has taken strong measures on security – mostly to reduce the selling of stolen iPhones.
(3)Check if the IMEI is Blocked--
Swappa has a free online tool to check your device’s IMEI number against the global blacklist. Go to the site, type in the IMEI number and wait a while for the results to show up.