Licensed individual that charges a fee to serve as the mediator between the buyer and seller?

Real Estate Broker.

Explanation:

As per the question, a 'real estate broker' would be described as the 'licensed firm or the individual who acts as the mediator or middle man between the buyer and the seller'. They function to arrange financing or settle contracts for their respective clients. These brokers acts the representatives of their clients and works independently on the basis of commission in return to arrangement of contracts and negotiation for their clients. Thus, 'real estate broker' is the answer.