The great majority of people cannot answer such questions unless they themselves are avid book collectors. So, we guide them to the best and easiest way to discover for themselves the value of old books. Go to your Public Library and ask for reference books which give the values of old, used, rare books. Finally, some universities have Rare Book Rooms with Curators in charge , or bookstores where these guides might be available for use on the premises. Your local bookseller is at a disadvantage here, by the way.
Reference books are costly and it takes time to price a stack of books.