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barrysloateYou are correct that most secrets will be kept secret by insiders. But a few thoughts nonetheless:
There are cabinets and CdV's of ballplayers in street clothes, but the only trick you have to know is what the players look like, and be able to identify them on the spot. For example, most cabinets of Harry Wright show him in a suit, but you have to know what Harry Wright looks like. Of course, every 1880's photograph features a man with a handlebar mustache, so you can't assume every one you see is Tim Keefe or John Clarkson. That is a common error made by everyone who haunts antique shops and flea markets. The first pile of cabinets they come across always seems to have a baseball Hall of Famer.
There are no great secret books as there will be something sports or baseball related in the title. You won't find great baseball content in a cookbook.
Your best hope might be that the seller knows he has something baseball related, but may undersell it. I've had that happen a number of times, but the so-called secret find isn't that easy.