My take on that is that they simply don't pay folks a reasonable price on a used gun. The price offered is almost insulting. Like cars, most people take the path of least resistance and with cars that usually means you trade as well as buy.
Never sell your gun to a dealer if you want top dollar or even remotely close to it. Just like selling your car to a dealer, only do it if it's worth it to you in speed and convenience. Otherwise sell it privately.