Stocks ended a choppy session modestly lower as the retail sector continued to drag on the benchmark indexes. Retailers such as Walmart (WMT) and Costco (COST) tumbled on signs of weaker sales over Black Friday and Cyber Monday. FitBit (FIT) was an outlier, though, climbing on signs of heavy demand over the weekend. Target (TGT) said the FitBit Charge HR was one of its best sellers on Black Friday, while Best Buy (BBY) announced that sales for wearables such as fitness trackers doubled from a year earlier. BHP Billiton (BHP) dropped after the Brazilian government threatened to launch a multibillion-dollar legal suit over a dam failure in the southeast earlier in November.