PP for a GT, Send the info to the account to the MM and the person sends the PP to the MM, The MM secures the account and gives it to the buyer and then sends the money to the seller.
So say I had MP, I would send my code to the mm and the other guy will give his account info to the mm, and the middleman will change the paassword so its secure and send me the info and send the MP code to the pp??! ;p
The MM takes the items being exchanged and when they are secured he will give them to the appropriate person. Usually a buyer paying with cash send the money or MP code once the MM has secured the item being bought.