This is incorrect. You agree to trade because you value the food (and eating the food) more than the money. The store agrees to trade because they value the money more than the food. Each side is more satisfied after the trade than before. Hence it's non-zero sum.
If they don't buy it from the store, they have to make it themselves. Everybody are satisfied in the transaction because they played well on this trivial zero sum game instead of falling into a negative sum swirl.
By the way, economic concepts are statistics. Arguments are only meaningful on large number of samples or over long period of time.
