Q: unlock

Hi! If your iPhone is locked by a carrier, you need to contact this carrier to unlock it. Note that this is only possible when the contract has been already terminated and there're no charges left to be paid.

If you Apple ID is locked, take a look at this article:

If your Apple ID has been locked - Apple Support

If your phone is locked with Activation Lock, you need to contact the seller and ask him to remove this iPhone from his list of devices. Otherwise, your iPhone will be a useless brick.

Find My iPhone Activation Lock: Removing a device from a previous owner’s account - Apple Support