I don't agree with those who say "Never sell a gun, ever".
Hindsight may sometimes haunt you but you get over it.
However, selling a family heirloom or something special to you should not happen and should always have a place in your safe and collectibles are also special but everything else is steel, plastic and wood. Easily replaceable.
Some people have a finite income.
Some people have limited storage space.
Some people just like to try different things until one just feels right.