Selling is different as you point out, but once again not insurmountable.
A separate concern is that some brands/products have distribution agreements allocated with different holders to the rights in Canada and the US. A US vendor would not be allowed to violate the distribution rights of a Canadian licence holder by selling in Canada. They would have to be aware of whether they held the rights to sell the product outside of the US. This would apply principally to some of the large European accessory companies, but there are US manufacturers who have sold the rights to their product to a Canadian distributor, as well.
Some of this stuff is simpler than people are making out, but there are angles such as this that must also be considered.