If you have questions about the value of your older books, or are looking for reputable book dealers, consider visiting:
Antiquarian Booksellers Association of America (ABAA) has a searchable database that allows a very refined search and features lengthy descriptions of the books offered for sale. Book dealers whose offerings are found here are members of the ABAA, a professional association which binds its members to a code of ethics for the conduct of their business.
Bookfinder.com is a web-based resource which draws information from online book dealers' catalogs. It is simple to use and it allows users to compare the book they have in hand to other copies of the same book which are being offered for sale right now. Descriptions of condition and edition are clear