Basically, they help the two parties not get scammed. Here, I'll post how Cosrob MMs (It's a bit outdated, but you'll get the overall concept): Remember, if you're planning to use an MM, only use trusted ones that are reputable. Also, if you're talking about OMMs, they are the most trusted MMs in Sythe, they completed a rigorous application that deals with handling many high amounts of money. More information on OMMs: http://sythe.org/showthread.php?t=1302650.
Wulfspade gave a great answer. If you're interested in more detail on the Sythe Market please refer to the link in my sig!
thanks for the answers, im interested in buying a diablo 3 beta bnet account... and i got scammed once alrdy. luckly i got my money back. but i dont want that to happen again.
Many MMs (me included) don't know how to do Diablo 3 trades. So, it's probably going to be hard for you to find a MM, perhaps there's another website that deals more with Diablo where you can find a trusted MM?