For example:
$0 - $20,000 - 0% (yes, poor people wouldn't pay sales tax. You could even make it so poor people get a discount, even.)
$20,001 - $40,000 - 5%
etc.
The above tiers would be defined by income plus assets times some interest rate, to represent potential gain on assets, if it were liquidated and invested. So if you had 1M in assets, and made $100,000 a year, your income would be $100,000 + 1M * federally defined interest rate.
The main difficulty with this would be that debit/credit cards would have to be issues by the government or there would have to be some collaboration between the government and banks in order to accurately determine purchases.