Most countries must use their own currency.
Canada is the only country with the privilege of choosing the currency we sell in.
Since Canada has 10% of the population, and historically has been less inclined to buy by mail order than Americans, we prefer to use a currency that most of the world understands and the only one which Americans , the largest market in the world, understand.
It should also be said that , unlike Americans, most Canadian sellers will ship internationally.
Note also, that when shopping on eBayCANADA both currencies show on the listing and there is no reason for Canadians to get confused.