Ideally, the seller should list the price + tax if it is physically sold in a store. This should be fixed (along with the tips culture in US).
Online retailers still have this problem. Sales tax is literally based on the zip code you enter during the checkout page.
So how can this problem be solved? The only solution I see is a federally controlled fixed sales tax similar to EU. But, that would be unconstitutional in US. States hold the power and additional municipal taxes are at the county level.
I think you're trivializing the matter. I've had several conversations with Europeans about it and we eventually come to an understanding that its not easy. On surface, it sounds simple if we assume is a simple nation-wide VAT tax.