The Smith & Wesson factory has an historian. For a fee you can write to the factory; and they'll research the records on the gun. In order to find out the current value you're going to have to research firearm auction sites like: GunBroker, and AuctionArms, and GunsAmerica. Around here, $700 to $800 would not be unreasonable for what you're selling - Especially with the original box and papers.
If, however, you purchased the gun in the mid 80's then it was probably shipped from the factory well within the previous 3 years. (I don't think that's worth paying $50.00 to find out.)