When I was collecting Civil War books, the price guide I saw many dealers pull off of their shelf was exactly the same price guide I could buy directly from the publisher. In fact I tols several dealers that there was a newer edition available and showed them my copy
My experience has been that the price guide publishers are interested in selling price guides and not in participating in some secret society that only allows dealers access to their information.
All that said, my guess is that low volume sellers are buying the same guides we can buy in stores. The higher volume dealers probably pay for some kind of subscription service that is not easily available to the great unwashed.
Bottom line, though is that they are price
guides.