My bad buys are a bobbin winder that doesn't keep good tension and a rotary sharpen that just takes the spurs off, but does not sharpen. Adding to the list: inexpensive/cheap pins and marking pencils.
Re quiltingshorttimer's message of being a ruthless cleaner: I am in the process of moving to a smaller house. I have all of my quilting projects, kits and equipment boxed up. When I get to the new house all of the boxes will be in a staging area to open one box at a time and decide whether I need to "keep an item" or "move it on".
I foresee a number of rulers going into the "move it on" pile as well as wrong fabric choices. I'm combining two houses into one, so I'm sure there will be a lot of duplicates. Many items were purchased early on before I understood the concept of purchasing quality items once rather than replacing cheap many times. It will be interesting to see how many items I bought at quilt shows (due to nifty demonstrations) that I forgotten I had and never liked.