Most trade is controlled by weathy trading clans which leverage their size and buying power for protection from nobles and trading rights.
These Merchant Houses were born out of the idealism of the High Kingdom when the monarchy attempted to erode the disparity between common folk and the aristocracy.
Currently, these houses are considered by noble clans as an evil to be tolerated out of necessity.
The basic foundation of a Merchant House’s wealth is commerce. A House in an iron rich area will bring help the noble of that area by transporting his goods to market (by buying them at a speculative price) and selling them higher at the destination. Similarly, he will bargain for goods at market to return to his home territory and sell at a further profit.
There is also significant profits taken in the loaning of money. A merchant must be careful, however. If it becomes too great of burden for a recipient, there is always the temptation to run the moneylender out of the area and default on the debt.