I reserve the right to be wrong, but Walter Block doesn't have to! Ceteris paribus, both parties to a trade are better off afterwards than they were before. Walter Block neatly explains what's up with praxeoloy and how things like a theory of value are arrived at praxeologically. Praxeology is the deductive, logical method taking in a starting point of unlimited human desire versus limited resources. The trade example above is the subjective theory of value at work, that each party subjectively feels like they have made a profit from the exchange. Maybe that's why both parties say 'thank you' afterwards! And so, to progress!
econ, philosophy and some forays into business and culture