Note: The Tomb Kings are the only faction that use resources for their mortuary cult mechanic (Edit: A similar mechanic called forging has only been added for dwarves in Warhammer 2 since it originally posted this answer). For all other groups, resources only increase the value of trade agreements. They are therefore generally right to think that these resources are not actually used by the trading partner, but not in this particular example. A master in the art of commerce, Kong Rong has two unique ways to further increase his commercial influence: by maintaining high populations and by concluding a unique diplomatic agreement that further increases the strength of his network of trade relations. Otherwise, the usual rules of diplomacy apply. Vassals and gentlemen have their own diplomatic attitudes towards each other and can manage relations, trade food, conclude trade agreements, lend and lend money and, in short, do diplomacy with each other and with others as usual. In both cases, you need to prepare for high costs – both options require that you properly encourage the other group by offering generous payments, items, or a lucrative commercial or diplomatic offer. These two options will also be quite out of reach if the group is hostile to you – these diplomatic options are only available to allies and best friends. The last and equally important diplomatic option is trade. Trade is divided into several parts.
You can send or receive a certain amount of gold from a different fraction as a one-time or long-term payment.