A middleman is a person who helps facilitate trades between members and avoid scams. There are regular MMs, and there are Official MMs (Official middlemen have the rank underneath their name). OMMs are VERY trusted, and have gone through a secure application process to show they are trusted. There are also regular MMs who are very trusted, but not quite so much as an OMM. When choosing an MM/OMM, make sure to go through through vouches, MM history, remarks from others, etc.
And I highly suggest using an MM for your trades, it helps avoid any scam possibilities. Good luck!