I sold a gun once. It didn't work right so I insisted that the dealer take it back. If I knew then what I know now I would have fixed it myself.
I also regret not buying a pair of 1903s which had consecutive serial numbers and had been completely rebuilt. They looked new. $750 for the pair. I'm not a collector. I want to shoot my stuff.
I think we all have those bang your head on the wall moments when we look back and realize what a wrong decision we made at a particular moment.