You could try taking the phone to an Apple Store, armed with enough evidence that you are the legitimate owner of the phone. If there is anything that can be done, that's the place that could do it. It's a long shot, though. Everything I've heard indicates to me that it is not possible to remove activation lock without access to the original owner's AppleID.
Edit: In your shoes, I would be looking for a refund. You have been sold something that is not fit for the intended purpose.