I can't think of any that I regret passing up, but I can think of two things that I wish I hadn't bought.
In a fit of cowboy nostalgia at one local gun show, I bought a single action Herritage, Rough Rider, .22/.22 magnum revolver with interchangable cylinders. What a major Papa Oscar Sierra!!!!!! I sent it back to Heritage twice but they could not make it work correctly. I finally used it as trading material and dumped it on some poor unsuspecting soul.
The other was when I was fairly new into semi-auto rifles in the fun-filled days leading up to Y-2-K. Ahhh, those were the days! But I digress.
At a gun show I was interested in buying up as much ammo as I could for a Mini-14 that I had picked up at the beginning of the 1st Clinton Assault Weapon Ban. I bought a bunch of ammo in order to fight off the rioting hords who would be swarming out of Portland to steal my food. I needed 30+ round magazines, and one vendor there had a whole bunch, really cheap. Cool! You know that old saying; "You get what you pay for."? Yup. Its true.
Anybody want to buy a gasoline generator. Hardly ever used. Cheap! Only 8 1/2 years old.
(http://i204.photobucket.com/albums/bb294/pioneer461/Tinhat.jpg)