Ok, here is how I interpret the statute in question:
If you have to register property in Maine, Maine is going to collect a sales tax at the time of registration. However, if you purchase the property outside of the state, and you register it outside the state for more than 12 months, or can present documentation of use of the property outside the state for more than 12 months, you are exempt from payment of the sales tax. This exemption does not apply to certain types of property, notably automobiles, watercraft, ATVs, snowmobiles and aircraft.
As to aircraft, they will be charged sales tax, unless the purchaser was a non-resident at the time of purchase, AND the aircraft was not in the state of Maine for more than 20 days (excluding time spent on repairs) during the 12 months following purchase. Of course, aircraft purchased by a non-resident are not charged sales tax. So, we must assume that the only aircraft that ARE charged sales tax fit one of two categories:
a) A resident purchases and then registers the aircraft, or
b) A current resident registers an aircraft which he/she *purchased outside of Maine * while not a legal resident of Maine, but then brings said aircraft into the state for more than 20 days during the twelve months following purchase.
Aircraft sold in Maine to non-residents are not charged sales tax.
Aircraft sold outside of Maine to residents who then register the aircraft in Maine are charged sales tax.
Presumably, though it does not say so explicitly, if you keep and register your aircraft outside of Maine for more than 12 months, and don’t exceed the 20 days during that 12 months, you won’t get charge sales tax if you then register the craft in Maine.
We assume that non-residents don’t have to register aircraft in Maine; usually registration laws apply to those resident in the state.