I was referring to used gear... but indeed.
And I wasn't so much referring to the law as how they enforce the law. Every little tiny cross-border bit isn't going to be evaluated by a CRA auditor.
In any case I think we've all determined that the best way is to buy local so there is no chance of having to deal with this sort of thing anyway

Used or new, doesn't matter, a purchase is a purchase. The only time I am aware that a purchase is exempt from being taxed is when it has already been taxed by the same jurisdiction. You're right in saying that the CRA isn't going to audit every border transaction, but that is a far cry from your claim of it not being illegal.
Buying used locally is my favorite way or purchasing, but like yourself in shopping for a Sigma 30mm, the Canadian prices are nuts. I ended up buying from the US for about $330, half of the CDN new price. I don't have warranty, but I can buy another one for still less than buying in Canada.

