The film, which has been valued at £10,000, was found during a clearance of a house and shows the band being interviewed in Cardiff in 1965.
Paul Fairweather, from Omega Auctions, said the lost reels were a “great find”.
In the footage, the band are seen joking with a journalist attempting to interview them, with John Lennon saying Paul McCartney has five children in Swansea and Ringo Starr joking their next film would be a Western. They also break into a rendition of There’s No Business Like Show Business, and pull funny faces throughout the interview.
Other footage, from 1967, shows spiritual guru Maharishi Mahesh Yogi and The Beatles being questioned about their adherence to his teachings.
John Lennon says in the clip: “Of course it’s not a cult and if we didn’t take it seriously we wouldn’t be here.”
A third sound recording captures Lennon giving an acoustic rendition of his post-Beatles song God, and has also been valued at £10,000.
Mr Fairweather said: “All four Beatles are in fine form throughout both of the Cardiff films, laughing and joking, while the interviewer tries to remain serious.The sound and image quality is fantastic. I expect these have never been seen since 1965.”