I am normally in the Cheapshooter camp, being pretty much anti-sell. There have been a couple of notable exceptions, though.
I used to have a Beretta 21A mouse gun in 22LR. I liked the gun and it was a neat, tiny package. But I sort of lost faith in it as a defensive or backup gun, and if you did not keep it meticulously clean it might turn into a single shot on you. I traded it plus $80 on a Mossberg pump shotgun at the evil pawn shop. It was a fun plinker when it worked properly and I sort of miss it, but do not regret the trade. If I get another, it will probably be in .25 ACP.
More recently, I bought a Savage Axis in .223 and then came upon hard times. I decided I would sell it, and replace it with a nicer bolt .223 when times got better. That is still the plan, and I should be able to replace it shortly. The $200 meant more to me than the gun at the time.
Those are my only two notable situations. Again, I normally don't sell because if something works properly that is valuable to me and you can't be sure you'll be as fortunate when you buy used - or even possibly when buying new (granted this is rare). So I can't quite say "never" the way Cheapshooter and others do... but I am at peace with what has happened. Anyway, that is my story on selling stuff.