Lucky you Patty! Rush has some good ideas. Ebay is always my first stop too.
My state library has a book in their reference section listing the last auction prices of books that have been sold. If your library doesn't have a copy, ask them to do an inter-library loan search for you.
Patty, Aloha from Cleveland. One other thing you might review is on this very website. Go to "The Study" link. Scroll down to the bottom, the last section "Miscellaneous". Then click on the Panama Hat, by the article "Collecting Mystery Fiction". There's a great article by Otto Penzler about the very subject. I attended the 2012 Bouchercon (annual mystery writer's convention) and saw a first edition of H.W.A.K. on sale for $250. TXS Lou