There are already a handful of mods that have an economy built in, or a trade system like you defined. My mod is actually one of them :P. I’m actually going to be reworking the economy system of it completely starting tonight so if you check out my page and let me know what you are looking for or how my mod can help you I would love some direct input on how you would use it. Steam Workshop::Kronos' Trade Center v1.22 (WIP)
Feel free to add me on steam if this interests you and we can talk more.
It will be with pleasure to speak with you about your Mod, I already sent the invitation to my steam. I will can explain you the Idea and we will see how to put this in your mod