If you are really worried about it, you can ask the seller if you can pay in Canadian funds. The approximate * exchange will be listed on any listing if you look at it on eBaydotCA.
It's fussy and not particularly useful for either of you, but many sellers will agree. The Canadian dollar payment still has to pass through Paypal's US bank on its way from you to your sellers however.
* Approximate, because the exchange rate is in constant flux , changing minute by minute, and will also differ from one bank to another. The rates posted online and in newspapers are for bank to bank transfers in the millions. More accurate for you is the rate at your bank branch, which may have changed by the end of the day.