Guide
How to Remove Find My iPhone Before Selling
Activation Lock protects your iPhone. Remove it through your own Apple Account before transferring a device you lawfully own.
Updated
Back up before changing accounts
Verify a current backup and make sure you can access your Apple Account and trusted phone number. If you are moving to a new phone, transfer important data, authenticator apps and cellular service before erasing the old device.
Turn off Find My using your own account
On the iPhone, open Settings → your name → Find My → Find My iPhone and turn it off with your Apple Account credentials when requested. Menu names and security requirements vary by iOS version. Follow the on-screen instructions and Apple's current documentation.
Allow time for security protections
Stolen Device Protection and account recovery can introduce extra verification or delays. A buyer cannot legitimately skip these protections for you. Resolve access problems with Apple before arranging the final transfer.
Verify that the device is ready
After following Apple's sale-preparation and erase steps, confirm that setup does not ask for your old Apple Account. Never provide your password to a buyer or use an unofficial lock-removal service. If someone else's account is linked, that owner must resolve it.
Official instructions
Consult https://support.apple.com/108934 for Activation Lock and https://support.apple.com/109511 for preparing an iPhone for sale. Port City Buyback does not bypass locks.
Ready when you are