Well.. it is someone who helps you in a Trade.
He is the MIDDLEman from the trade.
Well.. let's take a sample:
You are the RSGOLD Buyer, im the RSGOLD Seller.
We are using a MM.
I must give the RSGOLD to the MM , then you must give your payment to the MM.
When he got's all the required stuff he gives the RSGOLD to you, and if you were paying with PayPal , he gives the money to me.
Do you know what i mean?
-Yondaime