A cartel might work if the players involved had enough capital where they could buy-up all commodities priced bellow the rate they set. Those players would also have to be patient enough to hold all of those goods until prices have adjusted to the point where they're able to introduce them to the market.
So if you decide you want bows to sell for no less than 300g you would need maybe 75 to 100 million gold to buy all bows priced at or bellow 300g. You would then need to refrain from placing those bows for maybe 3/4 weeks during which you continue to buy anything at or bellow your set price. Once the market has adjusted (or if it does at all. There's always the risk that you run out of money before you've removed enough bows from the market to make a meaningful impact on prices) you could then dump everything you've accumulated at your set rate and hope that demand is high enough that you can sell everything.
Just for funsies: the total volume of bows bought/sold between 4/1 and 4/7 was 430.7 thousand. The average price for this week (once you take into the account the volume sold each day) was was 270g each. I would prefer not to do integrals on Easter so I'm going to take a stab and say that 70% of that volume was sold at a price less than or equal to 300g which leaves us with 301.49 thousand bows. Assume (lots of that going on! Doing the real math wouldn't be too hard but I studied Classical Literature in school and am feeling lazy today :P ) that the average price of this volume is also 270g. This gives a total price of 81.4 million gold worth of bows that are undercutting your base rate.
This is just 1 given week and I doubt that a week is enough time to raise the market price. More likely is that once you start buying up bows people who previously weren't listing them will begin to do so. It will probably take at least 3-4 weeks of dedicated buying in order to soak up enough of the volume so that prices really begin to rise in earnest. And even if you are able to do this it would cost you at least 350 million before you see prices rise and even then it would take forever to release your stock back into the market at a slow enough speed so as not to cause a crash.
So in conclusion: if a group of extraordinarily patient players were able to raise 350 + million gold they might be able to make a long-term change in prices for a single advanced res commodity (doing this with basic res would be next to impossible).
A far easier way to raise prices would be to convince a very large Alliance to declare war on another very large Alliance
Edit: I'm hungover on seder wine and can't spell!
Edited by Beecks - 09 Apr 2012 at 04:34