Well the basic principle of a MiddleMan is that they are the people that you use to help reduce the chance of a scam in a trade.
Take for example you are selling an account for money.
Person A wants to sell an Account to Person B for x amount of $.
Person A will give all the details of the account to he Middleman, and Person B will send the money to Person A.
When Person A confirms to the MiddleMan they recieved the money, the MiddleMan gives Person B the account.
This is only an example and there are many other factors that can go into play. But that is the basic principle of a MiddleMan. For reference, I would go to the MiddleMan forums here on sythe. They have stickies regarding who they are and what they do!
Best of luck!