best zip code for best price on used cars
where in the u.s. had the lowest price to buy used cars. this price should also
show any sales tax associated with the purchase
The reason the price doesn't show sales tax is because the price isn't the final number. It's just an asking price, plus you can purchase a car in one state and register it in another like you are suggesting on doing.
Sales tax aren't going to make that big a difference, as compared to regional difference in car values. Also doesn't matter where you BUY the car, you will pay the taxes on where you REGISTER the car, namely your home state.