i've only been collecting for a few years, so i never really had a chance to miss out on all the great stuff in the 80's, butg being in california i'm lucky enough to have a new ban every fer yaers or so that makes me regret not buying the neccesities.
lets see...
i regret not buying at leat one AK style rifle
i regret not buying a VEPR when i had the chance
i regret not buying a few more AR lowers before the ban
i regret not buiying another KEL Tec P-32 when i had the chance
usually, i'm pretty good about not passing up a good deal though.
on the other hand i've bought a few guns i didnt like:
Sprinfield M-6 Scout (horrible ergonomics and trigger)
Beretta 950 Jetfire, 25 ACP (my first gun, but a HORRIBLE caliber and lousy sights)
a CZ semi-auto, take-down rifle in .22 Magnum (the gun had all sorts of annoying "european" style safties and one of the $40 magazines malfed every shot. it also had no way to take it apart for cleaning. i ended up trading it for a Marlin Camp Carbine in .45 ACP. What a sweet gun! i took it to the range and got two inch groups at 100 yards with an 18 inch hold over! i nailed a golf ball at firty yards with one shot (using a red dot sight) what a great, fun gun!)