A real estate investor from San Diego, Calif., who wishes to remain anonymous, purchased the photo for $18,000, according to the New Hampshire auction house RR Auction.
The black-and-white image shows a young Diana lounging with her friend Adam Russell, described by Diana biographer Andrew Morton as the great-grandson of former prime minister Stanley Baldwin and now a deer farmer in Dorset.
The two were shown back in the late 1970s, while they were on a ski holiday together – "but absolutely nothing happened," according to Morton.
After her 1981 marriage to Prince Charles, Diana gave birth to their sons, Prince William, in 1982, and Prince Harry, in 1984, before she and Charles divorced in 1996. The following year, she died in a car crash in Paris.