Finally, I would strongly oppose including firearms more then anything else on your list, simply because I dislike the possibility of arming a potential enemy. Besides firearms are expensive, I can't store nearly as many as I can other items, and they're only good for big purchases. However, I might give a firearm to someone who was my ally or to cement a treaty. Selling a firearm gives away a tactical advantage, while giving it to an ally creates a more solid tactical advantage.