Xenophobia.
This is a country where many people seem to think that Hawai'i is a town in Kenya.
Americans have no need for any other market because their own is the largest economy in the world.
However, the GSP was introduced to encourage US sellers to ship internationally. EBay did not explain clearly that this was only a suitable program for items valued over $50-$100. And then they automatically signed up all new sellers, the ones least likely to understand about international shipping.
So sellers and eBay were inundated by complaints from Canadians used to getting a free pass on low-value and small imports, even though legally we should have been charged applicable duty and tax on any over $20 CDN (about $18USD).
So eBay's truly dumb reaction was to remove shipping to Canada under the GSP for items valued under $50 . Again they didn't bother to tell either buyers or the affected sellers about this.
Aaaand...... here we are.
So it's not the seller, probably.
If you see something you want, and the seller does not have shipping to Canada, use Contact Seller to ask if she will ship to you using USPS and First Class International (which might even be Delivery Confirmed, it was for a while there). If she says no, move on. But both she and you may find a meeting of true minds.