I bought a very long ruler that has the grit on one side that's supposed to sharpen your blade as you cut. Nope! It dulls and scrapes up the blade while making a gooseflesh-raising sound instead. Fortunately the grit is only on one side so I can still use the other; it's yardstick length & useful for trimming finished quilts, but I'm sure I paid extra for that horrible grit.
I also wish I hadn't wasted money on a spinning mat, now I just use a small mat on top of my big mat, but that "small" mat is larger than my spinning mat, so the spinning mat just lives in the closet, virtually unused.
I wish I hadn't bought most of my Omnigrid/Omnigrip rulers and had spend the money on Creative Grids or Olfa or other more clear-to-read rulers instead. All those thick yellow/green lines are really hard for me to see through.
I think that's about all that I really regret. I suppose that's a good thing, as I've spent a LOT over the years!