Mostly US sellers?
They may be carelessly giving International Priority shipping instead of First Class International Package/ Canada.
The difference in cost is both huge and not well presented by USPS.
https://postcalc.usps.com/?country=10440
If you wanted to send a tracked package to Canada (use 90210 as a zipcode) it would be easy to think Priority was the only choice.
To even find FCIP/canada you would have to choose Size and Shape and then scroll to the very bottom of the list to find the cheaper rate.
By contrast Canada Post, to their credit, start with the cheapest service.
https://www.canadapost.ca/cpotools/apps/far/business/findARate?execution=e1s1
And of course, there are many US and UK sellers who use the Global Shipping Program, which is a Seller Protection system that adds Canadian duty and sales taxes to the shipping cost. It offers nothing for the buyer.
Does not E Bay have rules on shipping preventing that
Sellers who inflate the cost of shipping to make their asking price sound low are stupid.
First because the customer will be annoyed when the inflated price turns up, which will lose them a lot of sales.
And second because eBay charges a 10% fee on shipping, as well as on the selling price. So they are paying more while losing sales.