I have been trying to not sell any, and I haven't in a while unless it was to trade up for something I liked more.
I have one gun that is absolutely positively not expendable. My grandpa died when my mom was sixteen, and when my grandma died a couple of years ago I got his old Winchester Model 20. Its an old single barrel break open .410 shotgun that isn't worth a whole lot (may 6-800 bucks) of money, but I would sell my car and both kidneys before I let that one go.
The only other gun I would keep without question is something for personal protection. Right now it is my G32. Soon it might be my new Kimber. Doesn't matter what, as long as I have one I can count on. (I prefer 1911s, though
)
Everything else can be replaced. My 870, 700, .22 rifles, Mausers, Mosins, handguns,...well, all of them can be replaced pretty quickly and easily.
As far as regrets....
Well a buddy managed to get his hands on my CZ 452 Lux and I doubt I'll get it back. I'm working on it, though.
. The others...
SW 28, Marlin 25mn, Colt 1991a1 (getting that one back soon, though), Ultra CDP II...the list goes on and on.