Before purchasing a used iPhone, buyers should verify that the phone is compatible with their service provider. Additionally, it is important to make sure the phone is not stolen, and buyers should confirm that the phone will work on their cellular network.
Prior to the iPhone 5 and LTE, iPhones operated on two networks: GSM and CDMA. GSM networks were only compatible with AT&T and T-Mobile, while the CDMA phones were used with Verizon and Sprint. However, it is now recommended that buyers purchase a specific iPhone model made for their service provider. For example, AT&T, Verizon or T-Mobile users should purchase the iPhone 6 Model A1549 or the iPhone 6 Plus Model A1522, while Sprint users should purchase an iPhone 6 Model A1586 or an iPhone 6 Plus Model A1542.
Use the Apple's Activation Lock status tool at iCloud.com/ActivationLock to verify that the phone is not stolen. The device's IMEI or serial number is required to check the status, so buyers must obtain the information from the seller first before performing a status check.
To confirm that the device can be activated under an account, buyers should call their mobile carrier and provide the IMEI or MEID number. The mobile carrier will run a check on their system to verify the phone's compatibility. Verizon users are able to check the ESN, IMEI or MEID number on Verizon's website.