It depends which currency the seller wants to sell in, I prefer to use
ebay.com and sell in American dollars. I find
ebay.com slightly more improved than the .ca site. When you list on .com if you pay the fee for visibility then other countries can see your listing like Canada. If you don't pay the extra visibility fee per listing your listing may not been seen on other
ebay sites. So it is a good idea to search each
ebay site for your stamps as some on .com may not appear on .ca whereas others will. So yes you have to search .ca separately to see all Canadian stamps on offer