A rare photograph of teenage Diana Spencer marked "not to be published" was auctioned for $18,306, CBS News reported.
The photo, which shows Spencer before she became Princess of Wales, has been the subject of recent speculation.
It is believed she is 17 or 18 at the time of the photo and it shows her reclining on a bed with a young man. It was "acquired" by a British newspaper just after her engagement Prince Charles in 1981.
More from GlobalPost: Rare photo of teenaged Princess Diana set for auction
CBS News said RR Auction said the photo sold to a San Diego real estate investor for more than was originally estimated.
According to CNN, the back of the photograph is stamped February 26 1981, two days after Spencer's engagement to Prince Charles. Experts said the picture was most likely taken a year or two before that.