If you are shopping around for a new car and happen to find an on-line deal from an out-of-state dealer that you just cannot pass up, the rules with the Department of Revenue have changed for the better. It used to be that if you purchased a car from an out-of-state dealer, the Registry of Motor Vehicles would have to collect sales tax based on NADA book values which could possibly be higher than what you paid the dealer unless that dealer was registered with the Mass. DOR.
Effective 11/30/15, out-of-state dealers are no longer required to register with the Mass DOR and if you traded in your current car to purchase the new, they will deduct the value from the purchase price when calculating the 6.25% sales tax.
Keep in mind that they have not yet updated their website but they are working on it.