A store that screws a customer loses a customer, and probably some of that customer's friends as well. A homeowner who refuses to pay for work done loses his workers, and the respect of some of his neighbours, perhaps. Keeping your word, on the other hand, ensures repeat business. This works online as well, where sellers, who value their reputations, don't generally just take the money and run. A seller who doesn't value his reputation doesn't stick around long enough to build up a good one.
David Friedman has a reputation too. Son of the late Milton Friedman, he is a professor of law at Santa Clara University and the author of numerous books, including the classic The Machinery of Freedom: Guide to a Radical Capitalism, a third edition of which is slated to appear later this year. His writing is clear and thought-provoking and a joy to read, and he's a truly wonderful speaker as well. Can't wait till tomorrow!