The population of Canada is around 33 million.
The population of the USA is around 320 million.
The world understands the US dollar. Americans understand no other currency.
If you are looking on eBaydotCA both currencies will appear on the listing.
If you are living and buying in Ontario, you may have to pay HST, by the way. Not all sellers are registered.
You can ask the seller to bill you in Canadian dollars. This may save you money or cost you money depending on the exchange rate at the time of billing.
