Remember, you are most likely not paying a "Sales Tax", but a "Sales and Use Tax".
Your state wants the money for the use of the item. For example, I live in PA and DE has no sales tax. If I go to DE and buy a TV, PA says I should pay the use tax on that TV to them. Realistically noone pays this especially if you pay cash for the TV, but by law you are supposed to file it.
It just so happens that in your case, it is a recorded transaction that they can trace. Years ago people used to lie about the price of a used car to reduce the tax. Now they are using the book value to calculate the tax instead of the purchase price.
2016 GT/CS. Automatic, convertible. So far 100% stock!