Examine the condition of the book. The better shape the volume is in, generally the more it will sell for. Check to be sure the binding is intact and look for missing pages, torn or missing or damaged covers, all of which will lower the value of even a very rare book.
Check for signs of value. First and limited editions of important or historical books are often highly sought after, as are books illustrated by famous artists, books that were censored or that may have had famous owners. Books printed using a new printing process, with special bindings, the author's autograph or an inscription from a famous person can also be worth more.
Determine if the book is truly rare or just second-hand. If a book is scarce, it can increase its value. ACRL recommends using Internet sites such as Alibris, ViaLibri, ABAA and AddALL that provide information on how many volumes of a particular book are currently for sale.
Research prices of the books you plan to sell. Visit rare book auctions such as Christies, AbeBooks and RareBookBuyer on the web and see what volumes similar to yours are selling for. Visit sites such as Faded Giant that provide guides to the value of rare and old books.