I used to feel seller's remorse.
Now, when I sell a gun, it is because I buy a gun I want more. I just sold a 1911 and bought two Glocks (24 and 30S). I am infinitely happier, and know that ditching that safe queen for two guns I will get more enjoyment from will make me happier.
Now, I really don't like the idea of selling guns without buying a replacement.
You can't own them all, and its a dynamic market. Funds are usually limited. Buying, selling, trading is half the fun.