Middlemans help Middleman trade in order to prevent both sides getting scammed.
For example:
Cash for Runescape account trade.
The Middleman would get the cash, and the Runescape account, verify the Runescape account, give the account to the buyer, and the cash to the seller.
MM's are regular users of Sythes, who MM, but pretty often to be any good, they're trusted.
OMM's are the Official Middlemen of Sythe, they go through an application, and the Head Middleman (Finn) and other Sythe staff check through this application. They are the most trusted middlemen.
Hope i answered your question.