The logic of calling a flat sales tax regressive is (I think) that poor people spend most of their income, while rich people save more, and thus pay a lower per

The logic of calling a flat sales tax regressive is (I think) that poor people spend most of their income, while rich people save more, and thus pay a lower percentage of their income. Agree that this isn't a great way to look at things. And that (as you say) there are often different rates of sales tax on potatoes vs yachts.