A middleman is someone who helps two people through a trade. They normally get used when the parties involved don't trust each other, so they turn to a more trusted member of the community to facilitate things. So, let's say you're trading 200m rs3 gold for 20m 07 gold but don't trust the other guy, and neither of you wants to go first. You would approach a middleman (who would send you confirmation pms and all that), give the MM your 200m, and the other guy gives him the 20m 07. Then the middleman would give you the 20m 07 and the other guy the 200m rs3, effectively acting a short-term bank. If you have more questions, feel free to add my skype.