I sell something when I loose interest in it.
Once upon a time, rimfire benchrest was my jam. Over time, I lost interest in it, and started shooting other things. Sold the guns collecting dust.
Decided I much prefer shooting revolvers, so I sold off quite a few of my auto pistols.
Decided I liked AR's better, so I sold my Ak's.
Frees up room in the safe, and cash in my wallet, to put towards things I'll enjoy more.
I'm less concerned with having "the most stuff" and more concerned with having "the stuff I like".
I've met people who's mantra is that you can never sell a gun. I don't get it, but you do you, man.