How to Turn OFF Find My iPhone if I don't Know the ID & Password? It's a Pre-Owned Phone, I bought it from a Market Place.

Seller forgot to reset the iCloud Account, and I lost the Owner Contact details...

I've seen many Bypass Tricks Online, Nothing worked for me...

Finally My hope is here...

is there any experts here, Know How to Bypass the iCloud Lock to Turn OFF the Find My iPhone.
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.