If You Are Buying a Used IPhone, Check Its Warranty Before Buying.
Buying a used iPhone can save money (I’ve never bought a new one). However, buying used items comes with its own problems. Be sure to check its serial number on the Apple website before purchasing. It will show you if the phone is under warranty or Applecare.
If you meet them in person, click on Settings, then General, and then About to get his serial number. You can also find it on iTunes or on real iPhone hardware (which depends on your version). If you are conducting a transaction online, ask the person for the serial number so you can verify it.
Before making your final decision, make sure you are not fooled by prices. Also, don’t forget to use iCloud to make sure your iPhone hasn’t been stolen .
Check your services and support | Apple