Scenario 1: Trading your 07 Gold for his PayPal cash
The middleman will be the one collecting your 07 gold[usually], he will hold the gold. Once he has confirmed that he has the gold in hand, he will proceed on to ask the buyer to pay you via PayPal. Once you have confirmed that you received the payment from your buyer[usually MM will ask for screenshot for confirmation] and if both parties have confirmed, the MM will pass the gold to your buyer. Trade done. Just a heads up, doesn't mean with a Middleman you will be safe from chargeback. That is out of the Middleman's capability.
Basically he acts like a person in the middle, hence the name - Middleman. Everything will go through him before the deal is done. In this case, gold will be passed to him first to get it secured and then the buyer can proceed on paying it and so on.