I've seen car dealers in MA use Montana dealer plates. At the very least they are cheaper, avoids $220 / year excise tax and registration fees vs. $25 for Montana. I don't think it helps with any sales taxes- dealers don't have to pay when they buy cars, unlike customers. It would help if dealer uses the car personal reasons instead of just having it for resale.

Hmm... maybe I should start a car dealer just to avoid taxes on my next car..

The Montana plate for owners reminds of registering your boat in Wilmington Delaware to avoid state sales tax. You had to create an LLC to do this I think.