Broker

A broker is a middleman who brings others together to bargain, or bargains for them.
A broker is someone who helps arranges transactions between two parties.
The broker is sometimes a mediator between a buyer and a seller. The broker sometimes gets a commission when the deal is completed. 
A broker who also acts as a seller or as a buyer becomes a principal party to the deal. 
A broker is different than an agent: one who acts on behalf of a principal.