iCloud: Activation Lock - http://support.apple.com/kb/HT201365 - This is a security feature, in part to deter re-selling of stolen devices.
Turn off Find My iPhone Activation Lock (formerly: Activation Lock: Removing a device from a previous owner’s account) - https://support.apple.com/HT201441 - If an activation lock was put on a device, only the owner of the account that applied the lock can remove it.
Get them to remove it or return the device for a refund. It is otherwise useless to you.