I think your seller is confused. The buyer doesn’t have the option of being part of the program. Assuming that the seller is in the US, the seller has to set up their listings to use the global shipping program, the eBay international standard program or set up their listings to ship directly to international buyers.
When you look at their listing what shipping option does it show to Canada? It’s possible that the listing isn’t set up with any options or that the item can’t be sent to Canada with the program they are using.