When purchasing used iPhone you have to make sure to check for a few things. Most phone are carrier locked and even unlocked phones cannot work across different carriers. There are two main cell systems that the major carriers use. AT&T and T-Mobile use the GSM network while Spring and Verizon use the CDMA network. Make sure that when you are purchasing the phone you set it up so that you can check the settings on the phone. Make sure that the carrier will match yours as otherwise you will not be able to use it. After that has cleared, check that the previous owner has removed their Apple ID from the phone. If the phone still has their info you will not be able to use it correctly and they have the ability to track and lock your new phone. Last, check the water damage indicators. On iPhones they are located inside the headphone jacks, the charging port or inside the SIM card slot. Good luck.