If you paid sales tax in one state when you bought a car do you have to pay sales tax on it again in the state you get the tags?

This depends on the laws of the two states you are dealing with. People used to try to get around paying sales tax by purchasing vehicles in states other than where they live. This has been halted in most states by requiring people to pay sales tax in the state they live in when they register the vehicle. In many states this also applies to vehicles purchased from individuals, which used to be free from sales tax as well. Usually a car dealer would not collect sales tax on a vehicle purchased by a buyer who lived outside the state where you buy it, because dealers normally do not collect tax and file reports for other states besides the one they do business in.