Check if the device is compatible with your carrier.
When buying a used iPhone, you want to make sure that it will work with your existing carrier. Otherwise, paying to unlock the device, or worse, switching carriers could be a real hassle, potentially costing you more money than you wanted to spend in the first place.
You can check the device’s compatibility in a number of ways. You can put your SIM card into the phone and see if you are prompted with a “SIM Unlock Code” notification. You can also get the device’s IMEI number and put it into IMEI.info to see if it is locked to a carrier. In the best case, the device will be unlocked and work with any carrier.