• Most comic books do not have proper UPC or ISBN barcodes.
• The Overstreet Comic Book Price Guide comes out annually.
• Overstreet's PDF version is DRM'd and Windows-only.
• The Bit Comic Book DataBase site (http://www.comics-db.com/) has no pricing data.
• ComicBase (http://www.comicbase.com/news.asp) looks promising but is Windows only.