What do MiddleMen do?

I'm knew to this site, (obviously, as you can see my post count,) and don't understand some things. So I figured to ask here, as it's coming from people who actually use this. So, what does a Middle Man do for our trades? Any response would be great.

Thanks.

 

a middle man holds both of the items traded, mostly in account trades person A gives him the account info he checks it out and person b sends the money and then the middle man sends both items to the people requesting them.

 

Oh, okay. Thanks for letting me know! Now I understand why some people charge fees for it.

 

Holds both items in a trade then gives cash to seller and goods to the buyer in most cases.