I have to say, I can’t see a way for the alliance trading to work without opening the door to a lot of cheating.
As an alternative solution, I do like the idea of a marketplace where you could trade resources with the computer (like the old heroes of might and magic). Ideally for swapping food/iron/crafting materials. I also think that the Alchemy Lab that Juzam proposed in the forum could be interesting as well for epic and legendary items. (note: if you like the Alchemy Lab idea, put up some likes or refine the idea with more feedback!)
It would be fun to do more with our alliances, I just don’t think that resource trading will end up being one of them.