An abstract artwork was gathering dust at a Goodwill store in Milwaukee until a curious woman bought it for $12. The bargain turned out to be a lithograph by the American artist Alexander Calder, worth $9,000.

It wasn’t love at first sight, though. Karen Mallet, a media relations specialist for Georgetown University, didn’t really like “Red Nose” until she spotted Calder’s signature.

"I thought, I don't know if it's real or not but it's $12.99. I've wasted more on worse things," she told AP. Her Goodwill loyalty discount card brought the price down to $12.34, so it was even more of a bargain she couldn’t resist.

Later, on the internet she came across similar lithographs by the unconventional artist. Works by Calder can be found at New York’s Whitney Museum of American Art and MoMa, as well as at the e Georges Pompidou Centre in Paris.

Mallet's piece turned out to be No. 55 of his 75 lithographs made in 1969, seven years before his death. 

A Goodwill spokeswoman explained that staff at its 2,700 stores tries to spot valuable objects and put them for sale on the organization's online auction site. However, they are not art experts.

"That's kind of part of shopping at Goodwill – the thrill of the hunt," communications manager for Goodwill in south-eastern Wisconsin, Cheryl Lightholder, told AP. "You never know what you're going to find."

Goodwill shopping is challenging, especially when it proves to be worth the time spent searching for bargains. Last month, a Salvador Dali sketch spotted at another Goodwill shop fetched $21,000, while in 2009 a painting by Maynard Dixon, donated in Goodwill in California, sold for $70,000. In 2006, Frank Weston Benson’s oil painting, donated anonymously in Portland, sold for $165,000 – Goodwill's record so far.

• A Permanent Economic Emergency