I agree with you. The only way to implement a national sales tax is to amend the constitution in part to eliminate the income tax AND property taxes. I would also amend the constitution to eliminate the ability of the congress to write tax law and set the national sales tax at say 10%. States and localities set their own rate. And that is it no other taxation.
Property taxes are not federal or state, but local counties and cities use that money....they cannot be eliminated for a national sales tax.
The IRS should be eliminated in favor of a national sales tax, but a residual staff would have to be retained to process the sales tax information and collection.