Nobody wants the stone. Silent is merely doing you a favor. I know for a fact Mithias and Bedivere would have killed him without a second thought. Bedivere himself can't use the stone, and Mithias doesn't cave to coercion or cowardice. Both of them would keep to their word if they said they would let him live. 

Maybe Mo will barter with him. Maybe you can play a solo with Magnus. Personally, I'm moving on.

Mithias doesn't believe Martin knows anything of value and has no interest in bartering with him. 

Does that answer your questions?