London A museum in the Netherlands said the portrait was not by Rembrandt, and the provincial auction house in England was only advertising it as a work by one of his followers - valued at $3,078.
But when 15 minutes of bidding on the painting ended Friday, it had sold for $4.5 million.
"I was shocked," said Philip Allwood, who had conducted the auction in the town of Cirencester, west of London.
"It tells you about the art market today. People are very prepared to pay big money for the right pieces, or what they feel are the right pieces," the auctioneer said in a telephone interview Saturday.
"The Young Rembrandt as Democrates the Laughing Philosopher," a 9.5-by-6.5 inch portrait of a young man, had hung in a local home for years.
The unidentified winning bidder may have concluded that it was a self-portrait by Rembrandt van Rijn, despite expert opinion.
The 17th-century Dutch artist painted a series of self-portraits. About 40 are recognized as his work, but others are believed to have been copies made by his students.
Allwood, the auctioneer for the Moore, Allen & Innocent, said the Rijksmuseum in Amsterdam and the unidentified owner of the oil painting that was sold Friday had concluded it was not by Rembrandt.
The auction house advertised the work as by a follower of Rembrandt.
Jan Six, a Dutch art expert with Sotheby's auction house in Amsterdam, said Sotheby's was an adviser for a potential buyer - who did not win the painting.
"Nobody pays 2.2 million (pounds, $4.5 million) for a follower of Rembrandt. If this was a known Rembrandt and was published in 20 books and had a great provenance it would go for 10 million (pounds, $21 million)," Six said Saturday.
He said the palette and pose of the painting were very characteristic of Rembrandt, and that the face was clearly his.
If the portrait is one day accepted as a Rembrandt, the buyer will have a bargain.
In January, a Rembrandt painting, "Saint James the Greater," sold for $25.8 million at Sotheby's in New York.