… is from page 145 of Deirdre McCloskey’s superb 2008 essay “Sacred Economics,” which is Chapter 6 in the 2008 collection edited by Sandra Peart and my colleague David Levy, The Street Porter and the Philosopher  (original emphasis):
Unlike stealing or taxing or highhandedly appropriating, exchange is a positive – not a zero- or negative-sum game. If Sir Botany must tempt the peasants with offers of educational services or consultation on interior decorating in order to get the barley, both he and the peasants are better off. If he just grabs it, only he is better off and they are worse off. If I buy low and sell high, I am doing both of the people with whom I deal a favor. That’s three favors done – to the seller, the buyer, and me in the middle and no one hurt except by envy’s sting. The seller and buyer didn’t have to enter the deal, and by their willingness they show they are made better off. One can say it stronger. Only such deals are just.