Washington shppers weren't waiting for Neiman-Marcus to open to buy merchandise in the late summer weeks, according to the Commerce Department.

During August, the government reported, area retail sales soared 15 per cent from the same month in 1976 to an estimated $918 million. And, for the first eight months of the year, area sales were up 12 per cent to $6.9 billion.

The year-to-year sales gaibs were exceeded, among metropolitan areas, only by Detroit - which was still recovering from an auto industry recession during early 1976.

With consumer prices up 6.6 per cent in the 12 months that ended with August, the recent sales figures reflect real gains in merchandise volume of more than 8 per cent for the month - the best gain for several years here.

Area department stores, meantime, reported a 16 per cent jump in sales during August to $117 million, compared with $101 million a year earlier.

Only in downtown Washington has the sales performance been week. Volume at three large downtown stores - Woodward & Lothrop, the Hecht Co. and Garfinckel's - dipped 2 per cent in August to $6.5 million. For the first eight months, sales at the three downtown stores also were down 2 per cent.