It was listed as: "Original 35mm color film TRANSPARENCY The Beach Boys 1965 w/ GLENN CAMPBELL"
The seller had provided this description:
"35mm color slide-original MASTER color film transparency taken circa February 1965-- in original mount.
This is from one of the rare dates when guitarist GLEN CAMPBELL was filling in for BRIAN WILSON (probably taken 2-13-65 at the Academy Of Music in NYC!)"
The transparency looks like the real deal possibly with the exceptions of both the date and location. A similar picture is in the book THE BEACH BOYS IN CONCERT written by Ian Rusten and Jon Stebbins, pg. 57, and is credited to The Beach Boys' concert in Buffalo, New York on February 22, 1965, at Kleinhan's Music Hall. According to Rusten and Stebbins, 3,000 fans were in attendance and "behaved well" during the show. However, after the concert ended, fans stormed the stage and tried to get to the backstage area to see their idols.
In any case, the concert photo (transparency) is one of the few photographs taken of Glen performing on stage with The Beach Boys (without Brian, of course).
The guitar does not appear to be Glen's own guitar; maybe Brian Wilson's?
(Note: the green lettering on the transparency was done by the seller. Glen is pictured second from the left.)