Make sure the iPhone isn’t reported lost or stolen.
If you want to buy an iPhone from another person, the first thing you need to do is check that the device wasn’t stolen or lost. If it ends up in your possession, you will likely lose your investment and the phone — or worse, be accused of theft.
Ask the seller for the IMEI number (Settings > General > About) and check it on StolenPhoneChecker.org or IMEI24.com. Either of these sites will tell you if the device was reported lost or stolen.