I see, thanks — no on the contrary: my conjecture is that the origins of our zero-sum thinking lie in our history as barterers. An exchange of goods can easily be seen as a zero-sum transaction. If I help you move a wardrobe, and (in return) you help me cut down a tree, we have no means of precisely equating the two, and so we assume equivalence, to the satisfaction of both of us. We don’t feel tempted to say that you still owe me because it was a big wardrobe and only a small tree. In bartering nobody loses or gains, which makes it implicitly zero-sum.
Now when money comes into the equation, the seller (by strong implication, if not in actual fact) makes a profit. If we are stuck in the formerly-barter-zero-sum mode, someone must therefore make a loss, and that can only be the buyer.