Depends on what you're selling and why. Selling because you need the money always sucks. That's just a net loss. There was a WASR-10 I unloaded in grad school to make rent that I still kind of regret, but not enough to pay what they cost today. I also had an utterly junk USGI 1911 - flaking chrome, aftermarket sights, best used as a produce blaster - that I got rid of about the same time for the same reasons, and that itch only went away when I got one of the CMP guns last year.
The wrinkle for me is that I keep my collection money separate from my other finances. Totally different bank accounts. This makes it a lot easier to keep money that I make selling inside the collection, and it also makes it a lot easier to justify impulse buying a $1k+ rifle to my wife, because she can easily see that it just came out of my gun accounts. So as long as the money is going back in there it gets a bit more complicated.
There was a period about ten years ago when I got serious about focusing my collection and upgrading to nicer pieces. I sold a lot back then, don't really regret any of it. Some of it got sold to afford better examples of the same gun (Blue Sky import Garand rolled forward into SG CMP rifle, etc), others got unloaded because they just didn't fit what I decided I was doing. I flogged off a Carl G. m96 because I'd decided that I was going to focus my German stuff on Oberndorf specifically, and while I still have an Oberndorf Swede on my to-do, the money that I got from that rifle helped me pick up some actual Oberndorf things that I was looking for. I think that's how I got my numbers matching Oberndorf Gew98.
These days, though? All the fat has been cut, and I make a point of not buying things that don't have a specific place in my collection. So if I had to sell anything right now it would sting in some way, and it would be something that I would ultimatley want to re-acquire because now there's a hole that it used to fill. Really, if I was going to try and drum up a lot of cash by selling - say for a big purchase - I'd have to redefine the collection and eliminate one of my minor points of focus. Probably the WW2 autoloaders, since I got all those for a reasonable price and they've gone WAY up in price, but damn that would sting. I'd have to be really gunning for something impressive for the main Obenrdorf focus where I NEEDED the cash.
Ironically one of the only things I could see myself doing that for - a Gew41(m) - would also be the capstone to the WW2 autloaders section so lol damned if you do, damned if you don't.