The margin is not based on the percentage of your income/wealth that you spend, it is based on the amount you spend. Like income.

For example, $0 to $50k per year spend is taxed at 0%, then $50k to $100k is 10%, etc.

Although a continuous exponential function is preferable than discrete cliffs.

Are you saying that is how VAT works? Because I have no idea how any could keep track of all that.

No, marginal sales tax is not in effect anywhere. Assuming electronic payments, I could see it being relatively easy to track everyone’s spending since bank accounts/credit cards/etc would be tied to tax ID #s. Of course, there are all sorts of issues with possible downsides and jurisdictional issues.