Although it is not mandatory to use a middle, it is recommended, as it will keep you much safer from a scammer.
Trusted Middlemen - Galaxywide
Dac Kain
Ezekiel
Guybrush Threepwood
Owyn Darklighter
Togan Jano
Vansic Helranth
Galactic Freightways Volunteer Middlemen
Jaghatai Khan
Other Volunteer Middlemen (and faction)
Also note that you can post here if you would like to be included in this list but you must:
A) Be a graduate from the academy
B) Have some experience in trading
C) Know how to middle (obviously)