hmm. i think we need to keep in mind that each resource (orcs, mana, seeds, etc) takes a special building to produce, which of course takes up an amount of space. part of the challenge of the game is to find the best utilization of your space. so if you have not enough orcs, the devs offer up orc nests in the MA or event buildings that provide them, and armories to breed them. mana and seeds have their own buildings so if you have an excess, the expectation is that you adjust your city to produce less of one and have space for the other.
by providing a trade/exchange space, you're defeating a lot of the point of the game in making you manage your resources. and you would potentially provide an area of exploitation for a person who maybe deliberately cultivates a lot of a single resource, knowing they can swap for the rest far more easily than actually producing it themselves.