In the case of books, http://used.addall.com can give some idea of the value. It looks at a bunch of different book-selling sites. The results for pre-1930 books are somewhat polluted by the many modern reprints, some of which are good but most of which are lousy. (For example, they may give you just the text without diagrams and the text has many, many typos.)
There are many sellers who either have no clue or are trying to steal. One seller on Ebay routinely offers items at ten times the price you would pay for a similar or nicer copy on Amazon. Sometimes he slips up and is low on his price.
Some billiard books don't have a "market" in that no one is offering them for sale right now and Ebay has one only every two or three years. An example of this is the book by Andrew Ponzi that has a price somewhere between $400 and $2000. That's the sort of book you hope you find at the used book store. I bought a small 1910 pamphlet for $10 that typically goes for $100 -- that was my best find. In case anyone wants the Ponzi book, I have one for sale.
I almost never look any more , but when I have, I notice there are a lot of sellers who never mention that their books or posters or pictures are reprints.
Not sure what the legality of that is.