The biggest art fraud in modern U.S. history was shockingly simple. Yet it went on for 15 years, duped some of the world's most sophisticated collectors, brought down a 165-year-old New York gallery, and brought in more than $80 million. It is also a cautionary tale for anyone thinking of dabbling in the art world.

"Art and jewelry are the last sort of bastions of unregulated business," retired special agent Meridith Savona of the FBI's Art Crimes Unit said in an interview with CNBC's "American Greed." "If you're a collector, if you're in this art world, it truly is buyer beware."

Glafira Rosales first appeared on the New York art scene in 1995, showing up at the venerable Knoedler Gallery with a painting that she claimed was the work of abstract expressionist Mark Rothko. And Rosales said there was more where that painting came from. Claiming to represent a mysterious Mexican collector known only as Mr. X, she said she was helping him unload a collection of previously unknown works by Rothko and fellow expressionists Robert Motherwell, Willem de Kooning, and Jackson Pollock, among others. If true, it would be a treasure trove of modern art.

"This art is some of the most valued art of the 20th century," said author Maria Konnikova, who wrote about the scam in her 2016 book "The Confidence Game." "People paid tens of millions of dollars for it, and we haven't had anything new in it for a while."

But in fact, the art Rosales was peddling was the work of Chinese immigrant Pei-Shen Qian, who had previously made his living hawking portraits on the streets of Manhattan. Qian would imitate the styles of the masters, then "age" his work using substances like tea or dirt from a vacuum cleaner. It worked like a charm, with one purported Rothko abstract selling for $8.3 million, and a fake Jackson Pollock painting selling for $17 million, before art experts began to realize something was amiss.

In 2013, Rosales pleaded guilty to fraud, money laundering and tax evasion, and was ordered to forfeit $81 million. To date, she is the only person convicted in the case. Rosales, who cooperated with authorities and was sentenced to the three months she had already served in jail, said she was coerced into the scam by an abusive boyfriend, Jose Carlos Bergantinos Diaz. He is currently in Spain, having managed to avoid extradition to the U.S. on a 12-count federal indictment alleging fraud, money laundering, conspiracy and false statements. Also indicted were his brother Jesus, who is also believed to be in Spain, and Qian, who fled to China.

The Knoedler Gallery and its director, Ann Freedman, who said they were duped along with everyone else, have not been charged with wrongdoing. But facing multiple customer lawsuits, the gallery closed in 2011 after 165 years in business.