I'm not telling you to break the law, but my understanding is that the eBay guys may be doing it by describing the item as a telescope. Not legal of course, and are taking a significant risk of it being opened and confiscated.
Again, I'm not telling you to break the law, but remember it is a US law, not Canadian. If you are personally carrying the item and traveling by car, you stop at the Canadian Customs when you leave the US, and not the US Customs. So in this case they (US) have no great way to enforce the law. It really is up to the retailer to verify they are selling to a US citizen, and not a Canadian. It is illegal for them to sell a restricted item to a Canadian.