Regrets, I've had a few.

1) Leaving the bulk of my Chinese Transformers with a "friend" when I came back to Australia. They included MP Grimlock, Megatron, a bunch of Henkeis and Classics as well as G1 reissue Megatron from Japan. The "friend" somehow managed to "lose" them.

2) Selling off my old G1 Transformers for a pittance as a kid at a 2nd hand market, including Grimlock and Sludge, Pretenders, Headmasters, Cyclonus and Scourge. Along with lots of vintage Star Wars figures.

3) Buying that Japanese reissue Optimus Prime with mini hot rod figure (can't remember it's name) at a premium price. Later when I wanted to sell it found it was unwanted by fandom.