Best Buy shares tumbled on Thursday after the consumer electronics retailer said that comparable store sales for the first nine weeks of the quarter fell 0.9% for its roughly 1,500 U.S. stores. Total revenue, which includes international business, fell 2.5% $11.5 billion. Competing on price to win customers ultimately hit Best Buy's margin. The company revised its forecast for operating margin saying it would now be 175-185 bps lower than last year's fourth-quarter margin. The Street's Laurie Kulikowski reports.