Ok, I understand I will be part of a simulated world, but to what extent is the world simulated?
The towns, and the whole Fable world, have a complete lifecycle, a full daylight cycle, people go off to work, kids go to school. This breaks down to the point where in some places, a ship will come into port laden with certain goods, these are distributed around the neighbourhood, prices are affected - a whole supply and demand mechanic! You as a player can be part of this, and start trading yourself, for example read this article.
A full commerce system manages all the trading and pricing of everything commercially available throughout the game world as supply and demand fluctuates.