Well i think to be a middleman you have to be very trusted and prob high post count. Also you need to apply for the position and you can only apply when there is an recruitment thread for middleman.
Middleman basically as the name implies is the inbetween person between the buyer and seller. Using a middleman usually reduces the chance of getting scammed (though not guaranteed).
Google Search Definition:
A person who arranges business or political deals between other people.