Double Taxation still happens...
I too live in South Carolina, and we have a bit of an odd system here...
You don't pay a huge sales tax upfront... there is a $300 limit on sales tax in the state. The kicker though, is that you pay an annual tax on all vehicles (just like you do on real estate). So, you save quite a bit of money on the upfront purchase, but you pay more over the long-haul because you pay property taxes every year on that vehicle. As the vehicle gets older, you pay less and less and eventually, after the car is 5 or 6 years old, the annual taxes are only like $100 or less. But annual taxes on a new car can be fairly steep... $800 or more on an expensive vehicle ($40k+).
The "Double" taxation comes when you move from another state... My wife bought her car in Ohio initially, where she paid the full sales tax, around $1500, when she bought the car. From that point on, she owed no more state taxes on that car in Ohio. But when she moved to South Carolina and registered her car, she had to start paying annual property taxes. So in essence... she paid the full tax up front, and a year or two later, she started paying it all over again, year after year.
The tax man will get you going and coming.