Where are you checking the exchange rate? The rate you are seeing may be the rate that applies to major players: banks, corporations, governments, and professional currency traders. As with all sorts of markets, big orders typically get better prices than small orders. When you buy a box of cereal from a retail outlet, you pay a higher price per box than if you bought an entire crate of cereal from a wholesaler.
From the point of view of the credit card or Pay Pal, they are providing you a service and they deserve to be compensated for it. Because exchange rates continuously fluctuate, they are taking some risk that the exchange rate will change unfavorably for them between the time you make your order and the time they carry out the currency exchange.
You could get around this by opening a bank account in the US, deposit some dollars, and use this account to pay the sellers on EBay. Or, if the seller is agreeable, you could pay them with a money order denominated in US dollars. However, at some point you'd still have to exchange your Canadian dollars for American dollars, and you'll still get a relatively unfavorable rate, unless you have enough money to exchange that you are able to negotiate a better rate.
In fact, you may find that the rate provided by the credit card or Ebay is as good as you can do as a retail shopper. If you went to a brick and mortar currency exchange you'd almost certainly find that the rate is even less favorable. Credit card companies trade LOTS of currency, so they get very favorable rates, and they sometimes share a little of that with their customer.