There is also a wealth of background information to be found, giving new perspectives. Cleverness and persistence are still required in using search engines or agents. Auction sites provide some additional information when a sale actually happens. Often, especially with better merchandise, bids never reach the seller's hidden reserve. Even when they do, you must follow the bidding for days or weeks until closing to see a hammer price.
Auction sites do show what is hot.
They are also sources of information on legitimate reproductions. On-line price guides for fine art have been around for several years. They usually require a membership fee. In antiques, such price guides are just appearing. The Maine Antique Digest prices database contains sales records for better merchandise with illustrations.