Princess Diana 'mystery man' photo sells for $18K A rare photo up for auction caught international attention earlier this month, showing a teenage Princess Diana lounging with a "mystery man." A photo of Princess Diana with a "mystery man" has been sold - to a mystery buyer! The never-before-seen 1970s image of the royal, which created a stir when it went up for auction earlier this month, sold on Thursday for $18,396, according to New Hampshire auction house RR Auction, which listed the photo. The buyer, a real estate investor from San Diego, Calif., has asked to remain nameless, the auction house told TODAY.com, though he "seemed very excited" to win the image. The bidding war over the photo - which started at $200 - got heated on Thursday night, with interest coming in from around the world. "It was extremely competitive and exciting," said Bobby Livingston, the Executive Vice President of RR Auction, noting the auction house originally expected the photo to sell for just $1000. We had multiple bidders - very fierce ones. One was a very well-known actor in Hollywood, another was a Russian billionaire. The intense interest in the photo was launched on Jan. 4 when it first went up for auction, as collectors realized the unusually candid shot of Princess Diana contained a mystery: who was the man she was lounging around with so casually, and why had a photo editor written "NOT TO BE PUBLISHED" across the top? Answers to those questions only heightened interest in the photo, which was sold to Britain's Daily Mirror in 1981, just two days before Diana's engagement to Prince Charles. The man was later identified as Adam Russell, the great-grandson of former Prime Minister Stanley Baldwin, who was said to be smitten with the princess. Princess Diana biographer Andrew Morton told The Guardian that the photo was taken on a ski trip during which Russell and Diana became close after they both suffered injuries and were forced to stay off the slopes. "They kept each other company while the other went skiing and, at the end of the holiday, Adam was smitten," Morton told the paper after the photo was released. Livingston says the drama around the photo made it the most expensive item of Diana memorabilia that RR auction has ever sold. "The international mystery of who the gentleman was and the iconic stature that Diana has made it become a compelling item quite quickly," he said. The fact that for whatever reason the photo editor wrote that it was not to be published, because it was not the image that Buckingham Palace would have wanted in that moment - and who was the young man in the photo... It kind of tells the whole story. More from TODAY: Princess Diana biographer identifies 'mystery man' in photo Queen's unflattering portrait unveiled after 60 years Prince Charles shares worries, hopes for 'future grandchild' See images from key moments in the life of Britain's Diana, the 'people's princess'.