There's a lot of regrets on things I passed on. Mostly it's the bigger things. A $400 item at $200, for example, that's a bigger miss to me than a $40 toy that's now $100+. HLJ's sales are actually great for higher end items. My biggest regret being sentinel's Zarak/Black Zarak for $75/$100. Also the Arcadia VF-0S for $250, and the Sentinel Dancouga for $220. I have both of those last two, but I paid $50+ more and I'd have liked to of them anyway.
I regret sometimes not getting into MOTUC. I passed on a battleground Teela last week for $20 I somewhat regret, but the face was a bit masculine for my liking. I'm probably going to try and build up and evil warriors collection,s since I had a bunch of hand-me-downs of the originals so I'm nostalgic about those.
There's some recent Mezco stuff I'm kinda regreting missing, like red Son superman and I'll probably skip the regular Deathstroke for theStealth. I may regret missing the Hot toys Hulkbuster when it was under $1,000, but it been so long the hype has died down, and the Sentinel one is already pretty cool.