If used guns are priced right, they sell.
True, but priced right is pretty subjective. It's the most basic rule of economics: The value of any item is whatever someone is willing to pay for it. Typically, what people are willing to pay for a used gun (excluding collectibles and true rare finds) is way too little to make the hassle of a sale worthwhile.
For example, For a time I was considering selling my Marlin 1894 for something that's a little cheaper to shoot. New, after all the taxes, and california BS fees, I was over $600 in the hole for the gun. Then, I upgraded the trigger (another $100+) and upgraded the sight with a Skinner sight (approx. $80). Even though I have less than 200 rounds through it and it's never been in the woods, I know that I would be lucky to get even $400 for the setup. I'm not griping, It's just the way the market is right now.
It's actually worth more sitting in my safe. It's possible that some day, within my lifetime, the Marlin 1894 could be completely discontinued. After all, they can't be that profitable to manufacture in comparison to all the plastic fantastic stuff being churned out now. If that happens, the Marlin will probably end up being way more than $600.
I'm not actually going to relegate it to safe queen status. I'm just going to finish off my reloading setup and safe money on ammo that way. But the point stands: Unless you desperately need the cash or room in the safe, don't sell because you're going to ultimately take a bath on it.