Actually, inland trade is not that worthless. The first trade building costs around 800 florins and if a province has around 2 trade valuables, you get around 40-50 a year which is comparable to that of mines and farmland in most locations. A trading post in all provinces with 2-3 trade valuables is not that bad since most of these provinces have sea routes anyway.

If a provinces has no trade valuables, no trading post can be made.