Political groups can agree on trade through diplomacy. A trade agreement between two Groups Tilea and Estalia is also very difficult to obtain as a trading partner for reasons I do not understand. 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). For all other groups, resources only increase the value of trade agreements. So you`re generally right to think that these resources are not actually used by the trading partner, but not in this particular example. 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. Each resource you export increases your group`s income, while each imported one increases your trading partner`s income. Unfortunately, you can`t decide what resources you want to send them and what resources you want to receive. In the late game, it causes situations where you can win 100 gold from the trade, while the other faction 600 or more.
However, it is always worth doing, because you will not really lose something and trade increases relations with a political group. 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. It also depends on the characteristics of the political groups. A group like Kislev, which has passive quality, is by no means to act. Animal men, greenskin and chaos warriors cannot act. It works in a rather strange way. By signing a trade agreement, you send all your resources to the group you are acting with (except those the group already has) and vice versa. A simple example will allow you to better understand: it is because of the Lore.
If you were an elf, you wouldn`t negotiate with a vampire.