According to this Wall Street Journal article:

"Brad Anderson, chief executive officer of Best Buy Co., is embracing a heretical notion for a retailer. He wants to separate the "angels" among his 1.5 million daily customers from the "devils."

Best Buy's angels are customers who boost profits at the consumer-electronics giant by snapping up high-definition televisions, portable electronics, and newly released DVDs without waiting for markdowns or rebates.

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."