I have traded or sold several guns over the years, and in most cases I don't miss them but there are a few I wish I hadn't.
Like the near mint condition Colt 1903 that was traded for work rebarreling a rifle. I owned it for about 3 weeks, long enough to fall in love with it but not long enough to get out and shoot it. Another customer of mine came in and talked me out of it for $150 (more than I had traded it for). Three years ago, a customer traded me a 1903 for some work I did for him. I needed the money more than the pistol, but I took it for old times' sake.
Like the 1909 Argentine Mauser barreled action I got in payment for a loan I had made. It was in 308, and the previous owner had fired lead bullets through it at jacketed velocities. To say it was leaded up is an understatement. No problem (back in the 1980s), just get some mercury and fill up the barrel and let is sit overnight. It cleaned up fine. Dropped it into a stock I had sitting around. Then my friend's son needed a deer rifle so I gave it to him. He and his dad went hunting that year, his dad died the next spring. It was worth it.
Like the Winchester 42 I got from the daughter of an older gentleman who was rapidly deteriorating mentally. I went shopping for 410 ammo and discovered I could buy several boxes of 12 or 20 gauge for the same price as a box of the little ones. I sold that one, and got a good price, but years later I got a itch for 410s and couldn't scratch it.
Like my 1871 Mauser that I picked up years ago because the owner didn't know what it was. I liked it a lot, shot it some, and ended up selling it to a friend in a bulk deal after I got divorced. For years I reminisced about it and the great times I had loading and shooting the old beast. Earlier this year, I got a screaming deal on a 1871 Mauser rifle just like the old one (Amberg instead of Spandau, but that's no biggie). Now I'm getting ready to relive the past.
So you see, some stories do have happy endings!