For those that do not want to keep an inventory, meeting people and learning all you can about the people you meet is a top priority. Mentoring is one way to meet people ... but can take more time and effort than is lucrative trade wise. A potentially more useful method is to contact and get to know people who do spend a lot of time mentoring. Then instead of offering to mentor people you see come in on the immigrant channel, offer to find them mentors. Nonchalantly mention what you do at some point ... those that stay are likely to remember your name. Make sure you write their name down and keep an eye on them to see if they pay. Chat with them occasionally, eventually they will need to make trades and hopefully you will be able to find them what they need.
Wander the land and chat with anyone you meet, as well as with the owners of buildings you see (even if the person is offline, they are likely to respond to you when they do log in). Watch all of the microphone channels, view the Trade boards, as well as all of the Trade sections at atitd.info. IRC can also be an excellent source of finding trades.
I used to keep index cards for each person I met. I would put down any information that might be pertinent: location, guild(s), resources the person liked to produce, resources they hated to produce, general level of technology, what disciplines they were mosted interesed in ... anything at all that might be useful in making a trade for that person.
There are quite a few difficulties with acting as a Middle Man. No active transportation organization has been succesfully developed. This means that you will have to work out a method of transport each time. While you can often just leave it up to the two parties you are interacting with, your value increases if you can somehow provide the transportation yourself (this has become especially true with the advent and maturation of the atitd.info trade sections). This all too often means you have to move the goods yourself ... which is time-consuming and limits your number of trades. The Middle Man is also less needed these days thanks to the increae of use the atitd.info trade sections have seen. Another difficulty is figuring what to charge (the best method is most likely to set a number of different percentages to charge each party based on different situations, though I never tried this myself to see how truly feasible it would be).
(See also: trade)