The devils are its worst customers. They buy products, apply for rebates, return the purchases, then buy them back at returned-merchandise discounts. They load up on "loss leaders," severely discounted merchandise designed to boost store traffic, then flip the goods at a profit on eBay. They slap down rock-bottom price quotes from Web sites and demand that Best Buy make good on its lowest-price pledge. "They can wreak enormous economic havoc," says Mr. Anderson.