The only problem that I see is restraining Congress from making the tax larger than 0.1% and spending the money before it is collected. Governments are notorious for raising the percentage once it is started.
Other than that, I think that it should include all sales, including the 401k. After all, if someone has $200,000 which is more likely, the tax is 200 dollars.
The sales tax on a low priced new car is much larger than that.