Activation Lock is designed to prevent anyone else from using an Apple device if it's ever lost or stolen.
Activation Lock for iPhone, iPad, and iPod touch - Apple Support
Activation Lock is an anti-theft feature that cannot be bypassed. Only the previous owner can remove the lock by using their Apple ID & password. Apple will not help you find the previous owner. If the previous owner cannot or will not remove the lock, then the device is useless to you or anyone else. Try to get a refund (through eBay) or dispose of the device properly. In the future, see the 'Check for Activation Lock before you buy a device from someone else' section in:
Activation Lock for iPhone, iPad, and iPod touch - Apple Support