Welcome to Clraik!
A middleman (MM) is a person, who is as the names says in the middle.
Let's say you have a seller, a buyer and a middleman. The seller gives the item to the middleman to hold until the buyer has given the middleman the payment. Then the middleman gives the buyer the item and the seller the payment. Since middlemen are trusted people (Clraik has official middlemen - OMM), they are a gateway to safe trading.
And yes, I'd say this site is reliable and safe. It is run by the best in neopets cheating on my opinion![]()