Unfortunately, you've either acquired an iOS device that was not prepared for resale, or have acquired a device that is indistinguishable from one that was lost or stolen.
Contact the seller and get the activation lock cleared with the original owner, and convince them that this device is yours and follow the instructions for remotely clearing this; that it's your device and not lost or stolen.
Or contact Apple directly, and convince them that this is your device.
There's no work-around for the activation lock other than the original owner or Apple clearing it, either — if there was, that'd just serve to make stealing iOS devices more popular.