Quote Originally Posted by Lusted
The game automatically sets up the best trade routes for you, trading the resources in a province with the provinces that will give the best price for it.
It's been a while since I looked (probably since RTW) but the city trade scroll tells you what you're trading and where.

There does seem to be a linkage between resources on the map and what you trade in your trade routes. IIRC, if you have more valuable resources in your province, your export trade routes are more valuable. You get a smaller fee for imports.

Also, I'm not sure what happens if you have more trade routes from buildings than you have resources available to export. I seem to recall that adding trade routes beyond a certain point does little for you.

Wish we had more definitive answers on this.