I got a similar letter from the state about two years after I bought my sailboat. Since I paid the sales tax the day I bought the boat I sent them a copy of the tax receipt with a suggestion that they check their own records before sending dunning letters.
Basically, you owe the Maine sales tax unless you paid CT tax. If you paid CT tax you can deduct it from the Maine tax bill.
Think of it this way, you got to delay paying your sales tax for two years. Consider it a zero interest loan from the state.