The nation's retailers reported modest January sales gains yesterday as an overload of outstanding bills and the absence of widespread price markdowns trimmed shoppers' ranks.

Analysts said the industrywide sales were mildly disappointing, but they added that January is the least important month of the year for retailers.

Sears, Roebuck and Co., the nation's No. 1 retailer, said its January sales rose 0.9 percent to $1.76 billion, from $1.75 billion in January 1985.

The second-largest chain, K mart Corp., reported a 6.6 percent gain to $1.26 billion, from $1.18 billion.

J. C. Penney Co., third among the retailers, said its sales rose 3.5 percent to $683 million, from $660 million in January 1985.

Jeffrey Feiner, retail analyst at Merrill Lynch, Pierce Fenner & Smith, said consumer debt load was the primary reason for January's "modest and somewhat disappointing" sales performance.

Alan Silverman of Evans & Co. pointed specifically to the rise in auto sales as the key sign that consumers' debt burdens have increased. "Making down payments and getting financing adversely affects people's ability to buy general merchandise," he said.

Walter Loeb, an analyst with Morgan Stanley & Co., said, "The traditional sale (markdown) period in January was not as strong as in past years, primarily because there was less merchandise. Retailers cleared their inventories at the end of calendar 1985."

In addition, Loeb said, "The consumer was overextended and wasn't willing to spend, even on sales."

Jeffrey Edelman of Dean Witter Reynolds said when auto sales begin to drift along or move down, consumers will rebuild their savings, accumulate debt at a less rapid rate and ultimately increase spending again.

Fewer available bargains also cut into shoppers' enthusiasm, said Monroe Greenstein of Bear, Stearns. "Retailers kept inventories in much better shape than in January 1985, and so there was much less carryover merchandise that had to be sold," he said.

Bernard Sosnick of L. F. Rothschild, Unterberg, Towbin said January's sales showed there was "very little buoyancy" in the consumer sector. He and other analysts cited a slowdown in spending among low- and moderate-income consumers, the primary customers of such mass-merchandising chains as Sears and K mart.

Federated Department Stores Inc., the fourth-largest retailer and the parent of Bloomingdale's and Sanger-Harris, said sales for the four weeks ended on Saturday were $598.2 million, down 12.3 percent, from $681.9 million during the five-week period ended Feb. 2, 1985.