I've only sold two guns in my lifetime, and both are recent enough not to have the sellers remorse yet.
I sold a M91/30 that never got shot, but as it was my 1st centerfire rifle, I thought it would have more sentimental value than it did. Got what I wanted for it, haven't looked back. no regrets.
The second I sold was my Hi Point 995TS carbine. It didn't get a lot of use, but I did enjoy shooting it. Sold it and between the two, purchased my S&W 469 and had money left over.
I sold two guns I rarely used and in return got a gun that will be my EDC with my CPL.
I have a few guns that would be classified under "guns I regret Not Buying"