To me, guns are tools and toys. I don't get emotionally attached to either. I've sold dozens of guns over the years because I either lost interest or found something I liked better. Very few if any regrets.
With that said, there are some decisions I made that made sense at the time, but I wouldn't make them again. If I knew then what I know now, I would not have sold:
1. A 4-inch Taurus M44 revolver in .44 magnum. I sold it before I got into reloading and learned that I could build lighter loads.
2. An accurized Remington 700 VSSF in .308. I sold it because I wanted a nice .260. Had I known then what I know now, I'd have had it rebarreled.
The only other gun I really regretted selling was a pre-lock S&W 317 kit gun with a 3-inch barrel and original iron sights--not the fiber optic/v-notch they come with now. I don't remember what I was thinking. But the seller's remorse made me buy another just like it when I had opportunity a few years later.