One of the most basic questions in business is this: Who is your customer? In the case of Visa (V), the dominant processor of debit card transactions, that answer is not the person who uses the debit card, nor even the merchant who lets the consumer use it. The customer is the bank that issues the card. By rewarding banks -- often to the detriment of consumers and merchants -- Visa has turned itself into a money machine. And you might want to consider investing in its stock.