Well, I looked at the Gun Traders Guide- but then I went and looked at some of the on line auctions that closed in the past 90 days- for guns that had sold. The book value for a light 12 in excellent condition is about 600. Ones that have sold, in varying conditions, run up to about $500. Right now times is hard, money is tight, and a lot of out-of-work folks are selling off guns. Expect a dealer to offer 50-60% of the book- they have to pay rent and light bills. You can also have a dealer sell on consignment- he gets a percentage of sale price (varies, 20% not uncommon, your mileage may vary) If a consignment gun does NOT sell, you will need to complete paperwork same as if buying it to get it back from dealer. .
You can also put it up for sale on an on line auction- I use Auctionarms.com, and gunbroker.com. If you do that, do not set a reserve, just start the gun at the price you could live with- have good pictures. Winner sends you payment, and info on a dealer (name, address, license number). You can verify license thru ATF website (called E-Z-Check) and you ship TO the dealer.
Do not try craigslist or a local ad. Scammers and thieves. Those are great shotguns- still have my Dad's.