It has been several years since I played Evony II, but I think that I remember that members of the same Guild (or whatever was called there) could give equipment to other members that were not as powerful as they were. This prevented the farming issue since the lower ranked members could not give to the upper ranked ones.

As far as trading cutting into their revenue they could charge the guild a fee for the privilege of being able to trade or create a Guild tech that would allow trading or the one getting the items has to use guild points to get them.