I imported a glider from Austria about a year ago. The State of New
Jersey didn't require me to pay sales tax, although they thought about
it for quite a while. I also recall that the glider was exempt from
federal import duty since it is considered some sort of appurtanence
of the aircraft. I've lost the links, but you can track down the
actual tariff schedule without too much effort.

Actually the trailer turned out to be a lot more trouble than the
glider. The NJ Motor Vehicles idiot insisted that I have EPA and DOT
paperwork on the trailer. I pointed out that the trailer doesn't have
a motor and she didn't know about that but said I had to have the
paperwork anyway. I had to be rather creative in order to get a title
and registration. The aircraft was easy in comparison. The FAA
really seemed to want to help; it was very gratifying.

Here look, I just found this:

Section XVII of the Harmonized Tariff Schedule: "Vehicles, Aircraft,
Vessels and associated transport equipment", which covers the glider
and trailer. Chapter 87, "Other trailers and semi-trailers", item
8716.40.00, is the trailer.

Chapter 88, "Aircraft, spacecraft, and parts thereof", item
8801.10.00, suffix 60, is the glider.