With the notable exception of No. 1 retailer Sears, Roebuck & Co., the nation's largest mass merchandisers reported yesterday that the key Christmas selling season began in November with modest volume gains.

Although retail industry leaders continued to express optimism about their business through December, there were indications that the seasonal sales volume won't measure up to previous expectations -- which will be translated into promotional sales and lower prices in future weeks, to get rid of excess inventories of goods.

Separately, the Commerce Department reported that overall retail sales in metropolitan Washington during September declined 2 percent from a record level in August, but were up by a substantial 19 percent from the same period a year earlier, partly because of soaring food costs.

Sales at Sears, which also is the largest general merchandise retailer in the Washington area, fell 6.4 percent in the four weeks ended Nov. 25 to $1.62 billion compared with $1.73 billion a year earlier.

Sears announced, earlier this week, that it will slice its advertising budget by 7 percent next year because of expectations that sales will decline next year to "recession levels."

In addition, Sears has adopted a new marketing strategy that places less emphasis on promotional sales and pricing cuts. "Lost sales are all in promotional items as we intended," said the company's new chairman, E. R. Telling, last week.

As a result of this deliberate cutback in less profitable volume, sales for Sears in the 43 weeks ended Nov. 25 were up less than 5 percent to $15.8 billion, a rate of growth that trails the inflation rate of 10 percent in consumer prices and indicating a reduction in actual merchandise sold.

Among other retailers reporting yesterday, the J. C. Penney Co. said November sales rose 14 percent from last year to a record $1 billion -- the first time Penney has topped the billion-dollar level in a single four-week period. Sales for the 10 months ended Nov. 25 were up a strong 20 percent to $8.32 billion.

Although Penney officers said yesterday they expect to continue outperforming national retail sales average gains, they conceded that a potential glut of merchandise troubled them in recent weeks. As the firm saw merchandise orders from its stores around the country building up in excess of sales expectations, "we began to communicate our concern to store and merchandise management," Chairman Donald V. Seibert said.

Speaking to analysts in New York, Seibert described his firm's objective as working out of a position of too much inventory without hurting sales momentum. Penney told stores to continue buying fast-moving goods while gradually working down inventories of slower-selling items, he said.

"We have made meaningful progress in correcting the inventory situation and expect to continue this progress through the fourth quarter," Seibert told the analysts, according to Dow Jones News Service.

One additional method being used by Penney and Sears to get rid of merchandise is to ask suppliers to sell the goods at discount prices to other retailers -- with the big chains paying the difference.

Penney projections for the retail quarter ending Jan. 31 call for general merchandise sales to expand 7 percent -- 5 percent inflation and 2 percent real gain. Home decorating and improvement lines, electronics, auto accessories, athletic apparel, outerwear and men's sportscoats are some of the best selling items today, Penney's said.

Among other retailers reporting yesterday, the largest increase during November was listed by Richmond-based Best Products, a catalogue showroom firm. Best sales for the month were up 30 percent to $91 million.

May Department Stores Co., owner of Hecht's, said sales in November were up 16 1/2 percent to $246 million (not counting new stores opened, sales rose only 7.4 percent) and sales for the 10 months ended Nov. 25 were $1.9 billion, up 11.3 percent (5.3 percent, not counting new stores).

K mart Corp. sales rose 13 1/2 percent to $999 million in the recent four weeks; Montgomery Ward sales rose 9.9 percent to $467 million; F.W. Woolworth sales increased 12 1/2 percent to $510 million; and Zayre Corp. sales rose 9 1/2 percent to $148 million.

Carter Hawley Hale Stores, owner of Neiman-Marcus and Richmond-based Thalhimer's, said November sales rose 9.6 percent to $214 million.

Government statistics for the D.C. area, meanwhile, estimated September sales of $1.06 billion for all retailers. For the first nine months of the year, area sales were $8.9 billion, up some 14 percent from the same period in 1977. Nondurable goods sales here rose 20 percent in September but general merchandise sales were up only 10 percent from last year.