>>>The 16th amendment must first be repealed before a sales tax is created.
I also think a constitutional amendment has to be passed to allow for the federal government to tax a purchase that does not involve interstate commerce.
I understand that the courts have been pretty liberal in their definition of interstate commerce, and they may eventually come down on the side of allowing the fed gov to do so without an amendment.
But it would likely take a couple of years for the cases to be resolved and during that time companies may opt not to impose and submit the tax to the government.
As has been pointed out, a sales tax is basically and excise tax and is covered in the Constitution.
The Congress shall have Power To lay and collect Taxes, Duties, Imposts and Excises...but all Duties, Imposts and Excises shall be uniform throughout the United States; U.S. Constitution Art I Sec 8.