Call a stereo store or pawn shop and ask if they have the Orion Trade In Guide. It'll have a low/high trade-in value, original list price and high/low selling price. Mint is no marks, like new and has the box, packing and owner's manual. If any of these are missing, the value droops. You can also go to ebay and click on "Completed Auctions" to see if any have sold recently.
