A luxury sports car sits on display in a dealership in Manhattan on November 30, 2017 in New York City.Spencer Platt | Getty ImagesU.S. retail sales rose moderately in August, driven higher by a jump in auto buying and healthy online sales. But there were also signs that consumers have become more cautious.The Commerce Department says retail sales increased 0.4% last month, down from a healthy 0.8% in July. Excluding autos, sales were unchanged for the first time since February.The modest slowdown follows signs that consumer confidence, while still strong, has slipped a bit as the U.S.-China trade war has intensified. U.S. businesses have cut back on their investment and expansion plans amid the trade war’s uncertainty and exports have declined. That has left consumers as a key source of growth.Online sales continued to soar, rising 1.6%, roughly the same amount as in July, when Amazon held its Prime Day.