In a way yes you'll be technically paying sales tax twice when purchasing a car out of state.
The DMV is hip to this avoiding their sales tax thing and will charge you the difference when registering your vehicle out of state.
So you won't really save anything by buying a car in another state and paying their sales tax on the vehicle because the difference will be made up at the DMV you register the vehicle with.
So you'll really be paying one tax but it will not be any cheaper and will seem like you're taxes twice.