Sales tax rates by state

[True Federalist (진정한 연방 주의자)]

Whatever the sales tax rate is, it'd be nice if posted prices included them here in the States. I just add 10% mentally to everything, but why not just post what you actually pay?

Two reasons.  First off, not including them lets a business post a consistent price without making less money in jurisdictions with a higher sales tax. Secondly, sales taxes (at least here in South Carolina) don't apply equally to all sales.  Some business-to-business sales aren't subject to sales taxes.  Also we have here a lower sales tax rate for people 85 and older. (Instead of the base 7% rate, they pay 6%.)

[True Federalist (진정한 연방 주의자)]

The US Constitution serves to discourage the imposition of state-level VATs.

Suppose the state of North Virginia imposed a 10% VAT on widgets.  A widget made in North Virginia, sold to a retailer in North Virginia, and then sold a final time to a North Virginia customer would be subject to the VAT at each step and thus it would be equivalent to a 10% sales tax on widgets.  However, a widget made in South Virgina and then sold in South Virginia to a North Virginia retailer who brings it across the border would not be subject to a VAT on that step, and the Constitutional ban on State import tariffs would keep North Virginia from raising an equivalent tax when it is brought into the North Virginia from South Virginia.  So North Virginia thus would be giving manufacturers of widgets made elsewhere a competitive advantage.  By contrast a sales tax is imposed on all widgets sold in the State and thus doesn't place State manufacturers at a competitive disadvantage.