I don't know who the seller is but I looked at the other autographs from the "Waterman's" book that he has listed including Calvin Coolidge, Orville Wright, George Eastman, Will Rogers etc. and they are all spot on , So the story seems to add up fine. Curious to me how the "likely not genuine" was rendered so casually if that was indeed the case.
Also in the 1930's and toward the end of his career Babe Ruth would pre-sign blank cards , like business cards but blank. And send them through the mail or even hand them out in person. The date of the Waterman's letter 1932 fits this time frame perfectly and the e-bay offered Ruth looks like one of those. Have attached a scan here for comparison of a genuine Ruth from the same era.
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jim@stinsonsports.com