A Middleman is used between two traders that don't trust each other. For example:
Person A is selling Person B 1m for $1, but they don't trust each other. So, they hire a middleman.
Person B gives $1 to the middleman.
Person A gives 1m to Person B.
The middleman gives Person A Person B's $1.
Hope that wasn't confusing >.<
Oh, and I think this is supposed to be in the blackmarket lounge, not RuneScape general..