Karen Mallet said she bought the painting at the Milwaukee store when she spotted the price tag and Calder's signature, and after some research discovered it was No. 55 of only 75 lithographs of the "Red Nose" abstract sketch made in 1969, WISN-TV, Milwaukee, reported Monday.
Cheryl Lightholder, communications manager for Goodwill in southeastern Wisconsin, said workers try to sort out valuables to auction on its website but some objects slip through and end up in stores.
"That's kind of part of shopping at Goodwill -- the thrill of the hunt," Lightholder said. "You never know what you're going to find."
Jacob Fine Art Inc. of suburban Chicago set the lithograph's replacement value at $9,000.