a mm is a middleman, usually some1 trusted that you use in a trade so nobody can get scammed, so say u are selling a lvl 100 to "jesus" for $20
jesus will give me (mm) the $20, then you give jesus the account, once he verifies to me that its all good i send you thge $20
if he tells me its fake or something wrong and proves it, he gets his $20 so he isnt scammed
hope that helps. any questions?
p.s if u are after a mm i can be one