This has been many years so take this with a grain of salt (maybe things have changed since then): I bought a Current Designs kayak in Canada, picked it up and brought it back by automobile to the US. If I recall correctly, I had to pay GST at the time of purchase and then go through a paperwork process to get a refund of the GST from the Canadian government. I seem to recall it was a bit of a manual/clunky process wherein I had to stand in line at the Canadian taxing authority (inside the duty free/border crossing), provide documentation, fill out various forms and then wait for my GST refund. I don't recall that it was a quick or immediate process of getting that money refunded.
As far as import issues: on another occasion, I had purchased a Canadian kayak and had it shipped. US Customs apparently informs state taxing authorities of such shipments and months after receipt of the kayak, my state tax office sent me a pretty harsh letter demanding payment of state sales tax.