How to Turn OFF Find My iPhone if I don't Know the ID & Password?
There's a good chance you bought a stolen iPhone. I'd bring it to an Apple Store or contact Apple support:

How to Contact Us - Apple

If it hasn't been reported as stolen, maybe they'll be able to help you. 

I would not expect that anyone here knows the answer to this. Even if there was such an exploit, knowledge of it could make a person quite wealthy. You aren't going to find out about it on a web forum or Google search.