The release of iOS7 has given iCloud users the ability to prevent anyone from erasing or activating their iDevices without entering their Apple ID and password first.
This feature is designed to prevent mobile devices from being stolen and it has helped to reduce iPhone-related theft in Major cities. However it turned out to be an issue for the iPhone’s second hand market. The feature must be disabled before a device is passed on or sold to another person. Failure to do so, renders the device unusable for the new owner.
From now on you can very easily check online if the device is locked before your purchase it. Just enter the IMEI or Serial number on this page. If the lock is ON you’ll need to make sure the current user disable it by going to Settings > General > Reset > Erase All Content and Settings. If the lock is OFF, you’re good to go as the device can be erased by anyone.