I've bought and sold many over the years. Try not to do either on a whim. Mostly don't regret my decisions, but there are a couple I regret letting go. That said, none of the guns I sold were "rare" to the extent that I couldn't replace them if I wanted to spend the money again. I'm with Cosmoline: they are tools, and I don't get particularly attached to them.
In a related vein, I've watched with interest as one of my older relatives has begun to cull his modest collection. He's in his late 70's, and still hunts regularly. But he "shoots" less. He's probably reduced the number by half, and doesn't seem to regret giving up any--especially those he's given to younger relatives. In the end, you can't take 'em with you.