The photograph might never have been seen publicly until now, RR Auction said.
Stamped February 1981 on the back, the photo was taken around the time Charles, the Prince of Wales, and Lady Diana Spencer ended months of speculation and announced they were to be married.
The photo came from the Caren Archive, a major private collection of rare newspapers and other publications, and was purchased seven years ago from the Daily Mirror newspaper.
Bobby Livingston, vice president of RR Auction, said Saturday that neither the auction house nor Caren Archive, knew the identity of the young man in the photo. However, he said he understood that a British newspaper had been able to identify him.
British media have identified the young man as Adam Russell, the great-grandson of former Prime Minister Stanley Baldwin.
In 1981, Diana first told news of her engagement to her friends, then moved out of her apartment Feb. 23 and into Buckingham Palace. What was widely labeled the wedding of the century took place on July 29, 1981, at St. Paul's Cathedral in London. Their marriage ended in divorce in 1996, and Diana died in a Paris car crash in 1997.
Another Diana photo to be included in the auction is an original 1988 news photo of the princess observing a marriage conflict resolution seminar.
The auction is scheduled for Jan. 17-24 in Amherst.