I say it depends on what you enjoy. Some guys enjoy just having a few (not only one..) guns and focus on being good with them - that's admirable. Others are collectors, and fill safe after safe with things they've never shot, multiples of the same thing with different serial number ranges, etc - I don't get it, but if it makes you happy, go for it. Others like me like to experiment and try different things. The only thing I really turn my nose up at is that guy buying stuff just because it's cheap, whether it's any good or not - at that point you might as well collect beanie babies or something.
Personally, while I do have quite a few guns that don't get used much, I enjoy working up loads and experimenting with different guns to see what they can do, what they're useful for, etc. I guess it's really about pursuing knowledge more than the gun itself. That's why I have something like 13-14 different cartridges in ARs, and multiples of many of them in different configurations, just as one example. (I'm sure the guys who only do ARs in 5.56 are shaking their heads at that, lol.)
But it's also about building skills to use a particular gun better, so I've got a few I shoot a lot, like 9mm Glocks, long range ARs and bolt guns, etc, while most of the others sit in the safe without much use once I've finished development to my satisfaction. Part of my rationale their is that the stuff that doesn't get used much, or isn't special in some way to me, is fair game to sell or trade towards the next interesting thing. I don't really have much interest in owning several hundred guns at once, as some collectors do, but doing this has allowed me to own hundreds over the past 30+ years or so. (Obviously I'm not one of those who'll never sell a gun - I've sold a lot of them and only regret a few; pretty good odds IMO.)
As for selling - with a few exceptions, most guns are NOT investments. Unfortunately, too many people think they are, and some of the guys talking about investments are usually the same guys bumping an overpriced ad fifteen times and grumping about lowballers. Someone above mentioned they'd rather lose a few hundred bucks and have the money to put towards something else - that's my approach as well.
An interesting (to me anyway) thing I've noticed about selling guns though - cheap guns usually get a lot of interest, but also a higher percentage of flaky people who won't bother to show up to a meet, offer half the asking price, etc. While more expensive guns, on the other hand, get a much smaller group of interested buyers but usually easier sales overall. And the line between those is lower than I'd have expected - a $300 Ruger pistol is squarely in that first category, but even as low as ~$750 for a squared away CZ or something has always fit that second category, in my experience. I've pretty much gotten rid of all my cheaper stuff though and don't have to deal with that anymore, fortunately.