... How does supply and demand work?
default prize of a fief is 15 gold
daily increase of S&D is determined by Prosperity .
Prosperity can be increased through spending of Production Points.
Prizes and Taxes can be increased or decreased through spending of Production Points
Fiefs with less then 10 gold per goods, i would categorize "buying fiefs", locations where you buy goods.
Fiefs with more then 20 gold per goods, i would categorize "selling fiefs", locations where you can sell goods.
When you buy goods, the available S&D is reduced by the amounts of goods you bought.
When you sell goods, the available S&D is reduced by the amounts of goods you sold.
This is independent to the prize, you can buy and sell either in "buying or selling fiefs", but well it may be not be economic but strategical depending how you interact with S&D.
If you buy/sell that many goods that the S&D would sink below the daily prosperity value, you got negativ effect on prize and i was also told on overall production, the later i can't quite confirm.
Best Case Trade Szenario:
Buy at a prize between 5 and 8 gold, in a way that the S&D is still above prosperity. Travel with those goods at least a distance of 55km to 60km for selling them there or swapp the goods with a trade partner half way. Repeat indenfenetly. Have for this 4 fiefs included, 2 selling 2 buying fiefs on each end of the trade route one selling and one buying fief close to each other.