Can I reduce sales tax from a new car purchase if I don't trade in a car, but sell it later myself?

I was reading this is possible but I don't know what form to file.  Essentially, this is doing what a car dealer would do when you trade in so you only owe sales tax on the total delta of the trade in vs. new car price.  ex: buy car for 50K, sell 'replaced car' for 20k privately, only pay sales tax on 30k.