[...] Credit-card companies are earning 3 or 4 percent on every purchase. That’s more than the growth rate of the entire economy. And it doesn’t even account for the primary source of credit-card company revenue, which is all the interest customers are paying (or further accumulating) on their balances. When a whole marketplace is not only paying up to the bank in the form of debt-based money but also paying a trusted authority to verify transactions, marginal costs become unsustainable. Merchants must mark up their prices to account for all the transaction fees, and commerce slows. Only giant retailers, with the ability to borrow money less expensively or even offer their own credit cards, are capable of reducing these fixed costs by filling some of the roles of the trusted central authority themselves.