Costco, the nation's seventh-largest retailer, has begun selling gasoline for discounts as great as 15 cents a gallon, investors have been told. The Seattle-based discount warehouse chain has opened gas stations offering discounted gas in Santa Clara, Oxnard, the San Francisco Bay Area, Albuquerque, Tucson and Scottsdale, Ariz., and the company is seeking permits to open gas stations at other locations, Costco President James Sinegal said at an investment conference. "I guarantee you," Sinegal said, "if we get in the gas business, gas prices will come down." He would not tell the conference where Costco was getting its gas. Tim Hamilton, a spokesman for the Automotive United Trades Organization, questioned whether Costco could consistently beat market rates by 15 cents a gallon.