I have an egg I'll sell for $43.
MMs hold the item, either on their account or by logging into one of your accounts, until payment is verified.
For example, if we were using video as an MM, he would log onto my account or I would sell him the egg in game. Once he has the egg (either way) he holds it until I say you have sent payment. This way, if you don't pay, he gives egg back, and if you do pay I can't run off and scam you. Simple enough?