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 conducted the auction.
"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 be copies made by his students.