I use both eBay.ca (I'm in Canada as well) and eBay.com. All I do is log in at eBay.com and list items there. Listings will be in USD but it but any withdrawls in Paypal are in CDN anyway. Only thing to watch if you sell in USD is to convert some money back in to CDN funds within PayPal (if your CDN balance is low). Ebay fees are withdrawn in CDN. I have had it happen to me that my Paypal gets put on hold because my CDN balance went below $0.00 when eBay withdrew money out if it.
If you are listing an item you have on your eBay.ca site, you will have to let the item run out (or cancel it) before you list it on eBay.com.
