There should never be a national sales tax unless the amendment making income tax constitutional is repealed. Just passing a law is not enough.
I'll be dead and reincarnated before the IRS is abolished.
one or the other but never both
Of course, the size of the federal government would radically shrink under such a proposal...
Correct. With an amendment that clearly sets out that the federal government can never impose both a tax on income and a tax on sales, the FAIR tax could work.
Yes, there are a lot of details that need to be worked out. But the disadvantages to individuals because of an income tax begs that a better system be implemented. The advantages of a sales/consumption tax is that, if you don't have enough money, you don't buy as much but you get your full income. You can put yourself on a spending diet until you have enough to get what you really want or need. You'll be able to buy a house sooner and faster. If you have a lot of extra money, you can go out and blow it.
A sales/consumption tax also forces the government to be more fiscally sane as their spending allowance will be directly apportioned by what has been sold. If they get out of hand with spending, people can stop buying and starve them out.
Like it'll ever happen, though.