You have to prove that the car had been registered in a different state to avoid the tax.
To elaborate, to avoid the Texas sales tax one would have to be a new resident, to have registered the car elsewhere before moving, and would have to pay an in lieu tax ($90 I think).
Some other states do let a resident avoid sales tax if the car is registered and used outside of the state for a specified period of time such as 90 days, 6 months, or a year, even if purchased while a resident.