Everyone has a phone that can manage calculations nowadays, few people can or want to calculate precise taxes. I still don't see how it makes advertising prices with taxes a "lemon market", which somehow prevents businesses from advertising such prices, if I understood your assertion correctly. Some businesses do advertise prices with tax, most don't. If people really wanted prices to include tax they'd naturally took their custom to the businesses that advertise "inc. tax" and took it away from "plus tax" ones, resulting in the situation opposite to what we have now.