A sales tax is regressive. The lower your income, the higher your tax burden as a percentage of income. Even the fair tax understands that which is why most versions of it has a huge tax credit per year.
The benefit is that you take the burden off the individual and leave it all to computers. There's a "Taxable" flag and the equation is simple. IF (item.category == Food or price < (select minprice from categorylimits where categorylimits.category = item.category)) then tax = 0 else tax = 0.25 * item.price
Done. What a beautiful world that would be.