The price of an item is determined by a balance between supply and the desires of those with purchasing power at each price.
The market essentially indicates the supply and gives a basic guideline for the prices as buying something for more toricredits than offerings on the market market would be stupid, as well as buying for the same price since it's more convenient to acquire items with a single mouseclick instead of bartering with users and executing a deal.
Once the supply drops the prices go up if the demand is unchanged simply because smart tradesmen see the opportunity.
Remember that an item is just as much worth as a buyer is willing to pay for. If you are a good businessman you are both willing and able to exploit that fact.
There is also a price optimum at which you can make the most profit as a salesman.
![](http://upload.wikimedia.org/wikipedia/commons/thumb/7/7a/Supply-and-demand.svg/500px-Supply-and-demand.svg.png)
D for demand, P for price, Q for quantity, and S for supply
Last edited by Sacrafan; May 23, 2013 at 10:13 AM.