Your food processor example is an interesting case for why I actually don't like markets as a rationing mechanism.
Let's say they're scarce in this scenario. Person A wants one as an assistive device, as you describe. Person B wants one because they prefer it to chopping by hand. Let's also say that person B has a lot more stuff worth trading than Person A, and is therefore able to outbid them on the market. Why should this be okay?