A tough year for U.S. retailers enters a critical stretch as Black Friday kicks off the holiday shopping season. On one hand, economic conditions are good. Unemployment is the lowest in 17 years, and credit is easily available. And shoppers are expected to spend more than ever before -- an average of $967 each, according to the National Retail Federation, up 3% from last year. But for the first time more of those shoppers say they plan to shop online than in big box stores like Walmart or Target.