An MM is a third person in the trade basically.
I'll give you an example.
Person A is selling a pin
Person B is buying a pin for Paypal.
The middleman, would recieve the pin from person A, and recieve the cash from person B. He would then check the pin was valid, and if it was, he'd give the pin to person B who was buying it, and the cash to person A who was selling it.
Middlemans just help out with the trade to make it go smoother, rather than people going first and risking getting scammed, but if you do use an MM, it's better to use a trusted/official, to cut down on the risk of them scamming.