tacomawv, use the online sites for getting trade-in and resale values for your truck: edmunds, kelly and others. With the rarity of regular cabs, I would not think you would have to much trouble selling your truck yourself if advertised aggressively (craigslist).
When I began searching for my regular cab, few dealers had them. The salesmen all said they can't keep one on the lot for long because they sell so fast. The Dealerships do not like to stock them because there are few options to run up the price so most folks settle for an access cab rather than waiting for a regular cab.
My dealer traded a dealer in WV for two regular cabs, mine and another. The other was sold almost as fast as mine. There is a big demand for them