Perhaps it's common knowledge, but it may be worth noting that (at least in the US) a merchant who accepts a counterfeit $100 bill, later notices that it is counterfeit, and does the "right thing" by reporting it simply eats the loss. The Secret Service thanks you, confiscates the bill, and leaves a receipt, but you lose the item you sold and the change you gave.<p>By contrast, a merchant who accepts a $100 counterfeit bill, does not notice it is counterfeit, and uses it to purchase something else keeps the profit and loses nothing. As a result, it is not in the self-interest of the merchant to look closely at any bills that have already been accepted.<p>If the goal is to catch counterfeiters, it would seem like there would be a better way to align the self-interest of the person who innocently accepts the counterfeit bill and the goals of the law enforcement agency.