I have never seen this image before. It does not appear in the Alfred Roller iconography (where Alma supplied the pics) and I have not seen it catalogued in any of the major Mahler collections.
The seller does not say where it came from.
Here’s the blurb from ebay:
THE EARLIEST KNOWN PHOTOGRAPH OF THE COMPOSER
An extremely rare early childhood CDV photograph of the great composer at age 3, shown wearing a checked coat and matching hat and riding a wheeled horse toy. A note on the verso in a contemporary hand reads: “Gustav Mahler—Iglau 1863.” 6 x 10.2 cm. Scattered ink speckles, central and diagonal creases, including small losses along left edge and to one location within the image. The present photograph predates by two years the first image of the composer recorded in Roller “Die Bildnisse von Gustav Mahler” and is thus believed to be the earliest known photograph of Mahler.
The seller wants $5,000.