Trade in ... activation lock

There is a carrier lock. Activation lock is:

Find My iPhone Activation Lock - This is a security feature, in part to deter re-selling of stolen devices. --> http://support.apple.com/HT201365

Turn off Activation Lock --> https://support.apple.com/HT201441 - An activation lock can only be removed using the log-in information for the account that applied the lock.

If this is a device for which you are not the original owner, get the owner of the account to remove the lock. If the owner has not provided contact information as a message showing in the login screen there is no way to contact the owner unless the person who gave you the device knows them.

If you are the original owner and have forgotten the log-in information, try to recover it:

- If you forgot your Apple ID [this is likely an email address you are using or used in the past] --> https://support.apple.com/HT201354

- If you forgot your Apple ID password --> https://support.apple.com/HT201487

Otherwise:

Apple may possibly remove a lock if a person can provide sufficient evidence of valid ownership. The device is normally taken to an Apple Store with identification and the original sales receipt. I.e., the receipt when the item was sold to the first owner, and preferably by that person. There is apparently no set policy (I asked at an Apple Store) and a store may simply refuse, particularly if they are not satisfied you have proven ownership by provenance.

Click on the link to read the article on Carrier Lock. "How to unlock your iPhone for use with a different carrier" - http://support.apple.com/HT201328 - Only some carriers offer the ability to unlock iPhone for use on any carrier. Follow the instructions in this article to unlock your iPhone.