Good deals:
a Colt New Frontier SAA in .45 Colt for $301. This was when the list was about $571. I sold it and made about $200 off it. Had I held it I could have tripled my money, by now.
Finding a customized LW Commander for $650, that jammed. All it took was a adjustment to the extractor and it ran like a top.
Finding a .270 Remington 700 Mountain Rifle for $350, with spectacular wood, very unusual for a Remington. It would string shots horizontally, and I discovered a metal chip stuck on the back of one of the Bolt locking lugs. Fixing that it started shooting better, but still acted up occasionally.
Bad deal - I sold it, thinking the action was sprung, then realized it was actually my wiggly Hoppes Benchrest causing the problem, so I sold it for no good reason. Whoever has it in California is one lucky SOB
Bad Deals:
Passing on buying an HK 91 for $450, back in the '80's. Not taking up a guy on trading a Uberti Rolling Block .45-70 for an UZI. (I had $200 in the Uberti). Later selling the Uberti for $250 - had I held it longer it could have brought $750 or so for Blackpower Silhouette.
Buying used Lyman dies - three sets, all had problems.
Buying used Contender barrels(two). Neither was accurate.
Buying a Kimber Stainless Gold Match, for list price, and finding it jammed and needed a trigger job.
And last, but not least, buying a SMLE #4Mk1 with a mint bore, and finding out the hard way that the bolt was broken and it had excess headspace