On the left (on your side), the interface shows you how much you earn with the trade and resources you export to your trading partner. The right side shows how much money they earn and what resources you import from them. It also depends on the characteristics of the political groups. A group like Kislev, which has passive quality, is by no means to act. Political groups can agree on trade through diplomacy. A trade agreement between two factions is because of the Lore. If you were an elf, you wouldn`t negotiate with a vampire. This is just another symptom of the larger problem called “Total War Diplomacy,” something that has shown very little love and attention since CA was first born. They even took poor little diplomatic boats and cars on the campaign map. They waved their finger against diplomacy and also took trade routes. Trade can take place between two political groups in campaign mode.
Trade agreements can be concluded on the diplomatic screen, allowing political groups to exchange trade resources and generate additional revenue for both factions. I have never seen evidence that more commercial resources would increase the likelihood of someone agreeing to a trade agreement. The diplomatic system is very veiled here. I have seen that groups with 200-plus relationships are opposed to profitable trade for them and that groups with 5-strong relationships accept the same trade. Maintaining friendly relationships with other groups offers a number of advantages. First, the trade agreement and the non-aggression pact that guarantees the security of your country. The Military Access Treaty allows your armies to move freely on the territories of another faction (which, without it, is treated as a violation of borders and deteriorates your relations). Tilea and Estalia are also very difficult to obtain as business partners for reasons I can`t understand. Trade is an exchange of goods that you can produce in certain buildings (wood, iron, exotic animals, etc.). You can act with another group if you don`t have the resource you`re acting on – otherwise they won`t be interested. Different goods have different prices, the more income from trade depends on the amount of resource you produce. Although diplomacy is often limited to other factions of the same race, you should take a moment to learn more.
Diplomacy allows you to conclude trade agreements, non-aggression pacts and even military and defence alliances (if your relations are good enough). In the case of fractions of the same race, you can also try to integrate them under your reign. Note: The Tomb Kings are the only faction that actually uses resources for their mortuary cult mechanics (Edit: a similar mechanic called forgery has been added to the dwarves in Warhammer 2 only since this response was originally published).