Stocks opened higher Monday, thanks to robust retail sales data. But enthusiasm quickly faded, leaving the major averages mixed. Weakness was mostly present in the tech sector. The Nasdaq turned a 0.5% gain into a 0.7% loss. Bellwethers Apple (AAPL), Amazon.com (AMZN), Google (GOOG) and Priceline.com (PCLN) were all lower. The index also fell below its 50-day line and the 3000 level, which acted as support last week. The Dow Jones industrial