Purchasing a used phone

Turn off Find My iPhone Activation Lock - Apple Support

RV9167 wrote:

What information do I need to know when purchasing a used Iphone ?

It's risky purchasing a used iPhone unless there is an option to return it for refund. Here's what you need to check:

• The phone is not Activation Locked. The only way to find out is to actually activate the phone using your Apple ID. If it asks for the password to a different Apple ID then it IS activation locked.
• The phone is not blacklisted as stolen. Again, the only way to verify this is to attempt to activate it.
• The phone is either not SIM locked to a carrier or is locked to the carrier you plan to use it with. You can contact Apple and they can check this; you will need the phone's serial number or IMEI.

Even then you are not completely safe. There is a scam where people will sell a working phone, then report it stolen to collect insurance. This will result in the phone being blacklisted after you have verified that it is working.