Agreed. I don't believe there will be a solution that is completely fair, aside from a 0 rate for such a sales tax. The other issue I have with the prebate is that a fixed cash payment to every household to cover the cost of the tax for "needed items" will put people who live in high cost states at a disadvantage. It will result in those people getting less value for their prebate than people in low cost states. That's another thing that exempting certain items from tax does address.
Please don't misunderstand me: I am not suggesting that sales tax OR the Fair Tax are without merits. I just would like to hear how sales-tax proponents adress the mother of them all --- the fact that poor people pay more of sales tax (in proportion) than wealthy ones. Aside from fairness, it is also a practical question: how can you expect to persuade people if you don't address their main concern?