I'll start with a few in both catagories.
Wish I HAD bought....
While stationed in Hawaii, I came across a Colt Python in a display case in a giant downtown mall in Oahu that didn't sell many guns. They had a few rifles and a dozen handguns. The Python was marked 50% OFF.
Driving by a yard sale, I noticed a guy had GUNS written on cardboard in the window of his pickup. I stopped and he had a Ruger Super Blackhawk laying on the hood of the truck he had parked in the shade, among the rest of the half dozen guns he was letting go. "Dad died and I can't have these in the house with the kids." The Ruger had factory Stag grips. He quoted a price and it was just a tad more than I wanted to spend, but Miss T told me to buy it. It was 44 mag, and I didn't reload it. "You won't shoot it much, but you always wanted one... buy it. You can buy a box of 44 now and then" I told the man I'd think about it and drove off. I came back by after lunch planning to buy the gun, but he said one guy bought all eight.
Bass Pro had a Collector's Edition Thompson 45 that came in a Violin Case with the drum mag and two stick mags.
The original price was really ridiculous but it had been in the store for over a year. I knew the GM at the store and we had met with him for something to do with the Veteran's Charity. We were walking out of the store together and had to pass through the firearms section when I saw it and I stopped to look at the gun. He called over the gun shop manager and got a "Best Price" on the package. It really was a best price, about what the Semi-Auto Thompsons were going for, just the guns. Miss T said it was up to me, we'd put it "on the card" and pay it off pretty quick. I thanked the Manager but walked out empty handed as the "card" was for emergencies. I always regretted it. I had looked for a Thompson ever since and haven't found another GUN at that price let alone that kit. The thing is, the violin case was really NICE, not cheesy. You really would never carry it around in that thing, but it would be cool to have in the loading room, or something....
The "Wish I hadn't LET GO..."
This list could be REALLY long, but I'll just list a couple two or three....
I had a customized Colt Commander, 70 series. Stag Grips, custom trigger, the works. A good friend, Navy Seal, was getting out of the Navy and going to work with the DEA as a boat commander in FL and loved that little gun, shot it better than I did. He made me an offer, I told him it wasn't for sale. He gave me the sob story that HE needed a gun he could trust his life to... and made me a much better offer. I told him it wasn't for sale. Then he cheated. He sent his wife over, with cash.
Model 27 Smith, shot like it was Laser guided. Got it from a mentor, who passed away. When I started shooting PPC, I need a PPC gun and with four kids money was tight. Really Tight. Got an offer on that gun I couldn't refuse. I should have.
Bicentennial Ruger Single Six with two Cylinders. Roommate getting out talked me out of the gun. Shouldn't have sold it. It was a shooter.
I could buy others, but that one was special. He loved it and wanted that one. Good friend.

