The Federal Trade Commission yesterday accused Sears, Roebuck and Co. of selling "down" coats that were stuffed mostly with feathers, and the agency sued Sears, demanding refunds for customers who bought the garments.

Sears responded with a strongly worded statement claiming it was a victim of wide-spread mislabeling of down items and had pulled the coats in question off the shelves a month before the FTC first raised the issue.

An estimated $75 million worth of down products are involved in the dispute, and Sears said it lost $22 million to $25 million by relabeling the items and cutting the price to reflect their true content.

The FTC complaint, filed in federal court in Denver, accused Sears of selling jackets labeled "100 percent down" that contained some feathers and selling items called simply "down" that did not meet federal labeling standards. FTC rules say "down" jackets can have as much as 20 percent feathers in them.

Down is the fuzzy, finest feathers of fowl and is prized for its insulating qualities. Bigger feathers don't insulate as well and don't cost as much.

The complaint against Sears is the latest in a series of actions brought by the FTB and some state regulators against makers or sellers of down garments. Named along with Sears in the latest case was Kellwood Co. of St. Louis, a down garment maker that is 30 percent-owned by Sears and sells 80 percent of its output to Sears.

The FTC charged that Sears' sales of mislabeled item was "unfair and deceptive" and said every single pseudo-down item sold constituted a violation of the law. The government asked for unspecified monetary penalties against Sears and a court order forcing Sears to comply with labeling requirements.

Sears said it first learned of the government probe of down labeling in December 1978, a full month after it voluntarily had removed all of its down merchandise from sale.

Sears said it learned of the feathers in its fillings from the U.S. Customs Service, which inspected the items because the import duties are different for down and feather-filled items.