Not quite regret since the purchases made sense at the time and I used them, but I've got a ton of fluourinated waxes and pure fluoros from when I was racing XC skis and got deals, and am nowhere near using them up since I've stopped racing and fluoros are increasingly being banned.
I guess I bought too much.
In general I tend to overbuy things, especially when I like them and can get them at good prices. For example, I've got many many baselayers, some new in their bags, that I like, but newer/better ones have since come out, so I'm not using them all up.
I also have several pairs of gloves I got when I got serious into alpine skiing that I like, but should have bought in a larger size. They fit and work fine, but would more flexible in terms of temperature ranges if got them larger so I could more easily use liners in them. I can just barely squeeze in very thin liners.