Apple introduced the Activation Lock feature in iOS 7 to help deter phone theft—if you’ve signed into iCloud and enabled Find My iPhone/iPad/iPod Touch, you can’t erase your phone without entering your account password first. Even if a thief put your phone into Recovery Mode and wiped its data manually, you need to enter the previous owner’s Apple ID password as part of the first-time setup process before you can actually use it. Activation Lock renders a stolen iPhone much less useful to someone who intends to use it themselves or resell it.
The feature can be misused, though. If you sell your iPhone to a legitimate reseller and forget to wipe it first, you might leave that seller with an unsellable brick that’s useful for parts but not much else. For buyers and sellers who want to check a device’s Activation Lock status without having it in-hand, Apple has just introduced a new Activation Lock status page that will tell you whether any device is locked if you punch in its serial number or IMEI.