...still argue that humans should not interact with each other...to trade. See Don Boudreaux's post What’s Central to the Case for Trade?
Trade comes naturally to children the first time they play together. One child recognizes that the other child's toys could be enjoyable if only they were to gain access to the toys. At first, the child follows a primitive notion and snatches the toy, until the parent (negating the need for years of game theoretic interaction) says "No, we share." The children benefit from each others' toys without having to purchase the toys themselves. They trade. They gain.