Brisk business at the department stores and auto dealers helped retailers enjoy a solid 0.8 percent rise in sales in March, the fifth straight monthly increase and the strongest since December, the government said Wednesday. Retail sales totaled $130.7 billion last month, according to a preliminary report from the Commerce Department's Census Bureau. Sales in March were 5.3 percent higher than a year ago, and sales in the first quarter of 1988 were 1.7 percent higher than in the final quarter of 1987. All figures were adjusted for seasonal variation. The March increase was the largest since retail sales jumped 1.1 percent in December. The last downturn occurred when sales declined 0.8 percent in October.