The GOP proposal is stupid, I'm certain of that and I haven't even read it (I imagine it takes an axe to the current system with nothing viable in its place).
But the system I'm trying to semi propose (I just wouldn't mind it as best I can tell, I don't know enough about any of it to really know) goes like this:
All purchases are subject to a federal tax (Buck said 15%, but I'd guess a bit higher, somebody with more econ knowledge can look into that more)
All adults/citizens/legal residents (there can be plenty of debate over who gets included) get a refund to cover their first X dollars of taxes from spending on the year.
Say we want the first 20k anyone spends to be taxfree, then if the tax rate were 20% we'd give everyone a $4,000 check that year to offset the sales tax.
No income would need to be reported at any point, and all taxing would be at the point of sale.