No one makes a trade unless he considers what he will receive more valuable to him than what he will pay. That’s why voluntary trades are always win-win: each party ends up with something more valuable than he’s traded away.

Sometimes the benefit is simply a good feeling, such as when someone buys something from a poor salesman just to encourage him, or when she donates money. Here the good feeling and personal benefit are worth more than the money given.

This helps us answer the question, “Why would somebody voluntarily give up something she values more in exchange for something she values less?”

Answer: She wouldn’t

Contrary to the famous line in the movie “Wall Street,” capitalism is a positive-sum game where all sides win to the benefit of society.