Which Western states have laws that charge sales tax on the difference between
a new car's price and the trade-in's value?  I know California charges you on 
the full value of the new vehicle regardless of trade-in.

If you are a California resident, is it legal to buy a car in a state other than
California without also paying California sales tax?  How does California 
enforce any law that requires you to also pay California sales tax (on top of 
the out-of-state tax)?
