For what its worth:
I live in washington. 2 years ago i bought an SXR from Canada. I drove across the border to get it from the seller. Upon returning to the border, i declared that i had purchased the ski in Canada and was bringing it into the US for pleasure use. I did not have to fill out ANY forms at the border, and did not have to pay ANYTHING to get it in. The border agent had me go to the border patrol office where they just double checked the bill of sale paperwork and title/registration (Canada has funky registrations imo) of the ski of the ski i had just purchased. After 20 minutes i was on my way, no issues at all.
I did not have to fill out any EPA paperwork or sales/tax paperwork to bring it into the US. I then went to my local DMV and transferred it into my name just like any other purchase.