Canadians are the only sellers who can choose the currency we list in.
Most choose to list in USD because Americans not only have 10 times our population, but a higher average income and the world's largest economy.
Until a few months ago, the loonie was higher than the USD, so the thought that sellers use the foreign currency to make extra money is unlikely. It does happen at the moment, of course, but I've been selling here since 2001 and sometimes it's up and sometimes i'ts down.
At the moment, I've dropped shipping charges on my items that can go LetterPost, because the exchange difference pays the postage.