I think a tax system this simple would be similarly simple to audit. Or maybe you would calculate your consumption tax at the end of the year, and then it wouldn't matter who you buy from.

I was thinking "black market", not "hard to administer". For that much money, there are plenty of people who are going to be willing to do off-the-books transactions.

And no, it won't be simple to audit. Can you prove that all the food you ate last year came from the grocery store? That you didn't buy any from your neighbor's cousin, who works at a wholesale warehouse?