If you think about it Fertile Lands is a form of market manipulation. So would be sending all trade fleets of a guild, say ATC for example, all on the same trade route. It would be foolish as far as getting plundered is concerned, but a lot of us have done it in the past. Especially when the ports are close together such as the Tortuga - Tzogos route. No one even has to say lets all go here but the profits generated determine where we as traders go just the same. I wouldn't say it would be possible to keep a particular resource pegged at the low end or the high end of the price range in a port for longer than 3 days without the use of fertile lands, which is being removed from the game any way. The only way the market might be manipulated would be if there were only a limited amount of bankers allowed in the game. Then they could collude like gas stations, in setting the same price for a gallon of gas. Except, in this case it would be interest rates on loans or the price paid for a bar of gold. I really don't think the seller would have any say if all the banks all stood together. But that doesn't happen in the real world with OPEC. Someone always wants to sell by volume instead of by quota throwing the whole price structure off. If everyone else was buying gold at 100 gp per bar and Captn. D was buying at 99 gp per bar I would go to him and so would most everyone else. He would make up in volume what he would lose by having the higher price. He might even charge different interest rates for loans depending on the risk involved with the player he is loaning to.
I guess what I am trying to say here is that there are just too many variables that stand in the way of "price fixing or market manipulation". I think greed will win out over the ability to fix prices for even a small amount of people. It's just human nature, or Keynesian economics...Eeeww, I can't believe I said...zzzzzzzzz