SAN FRANCISCO—Halsey Minor, the founder of Cnet and a collector whom Sotheby’s sued in September for $16.8 million, is the lead plaintiff in a class-action suit being brought against the auction house.

In a brief filed in a federal court in San Francisco, Minor and his fellow plaintiffs allege that Sotheby’s fails to disclose its financial stake in the artworks it sells.

In May, Minor purchased pieces by Edward Hicks, Andy Warhol, and Childe Hassam at Sotheby’s, and in September the house sued for $16.8 million, a figure that includes the $13 million he bid for the works, plus interest and legal and late fees.