We Need More Writers!
It is true that individuals can have a massive impact on the economy, but the will of the many is far superior to even the richest trading post gurus. When millions are playing, their collective decisions are what make you gold, not your Ebeneser-Scrooge-Tycoon-Style-Monopolizing.
Instead of looking for markets to "control" or "dominate", look for other ones you can ride all day long for easy money.
Take, for example, my post on platinum ore. That was a great example of just riding a good thing. I didn't sit there and try to create walls for people to undercut then buy them out (I probably should have in retrospect and bought everyone out lol) because I was more concerned about riding the market like a surfer rides a wave. But the moment you think a market is yours, a thousand people arrive and destroy it for you. I don't care if you have thousands of gold, it isn't enough to control most good markets (believe me). Maybe some shady low level weapon market, but the reward isn't worth the time spent controlling such things.
Eventually that wave becomes a small ripple and you move back out to the ocean for another wave. This is how you should place when you're looking for investment opportunities, not massive speculating on patches or whatnot. There is a place for that, and there is also a portion of your money that can go to such activities, but don't go crazy and invest everything in the next "wave."
TLDR: Controlling markets long term is foolish, short term is occasionally profitable but high risk.