Hello 'allison',
Do you have a US ebay ID? You don't really need one, but I suspect if your US mailing address
and a US ebay ID were together you would have had no problem.
Assuming, of course, that you have not been blocked for other reasons.
If a seller chooses not to ship to a certain country, that is his choice. If a shopper from that country
bids anyway, the shopper could be reported as the unwelcome nuisance the seller believes she is.
If the seller has ignored your pleas and explanations, and you are unable to bid on the item,
even with the US mailing address, - the seller has every right to exclude you (or anyone else).
You have said your piece to the seller, made every effort to accommodate his restrictions, so now
you must find another seller.
Further nagging will certainly land you on his personal blocked list from which you may not be
removed if he ever changes his mind and decides to sell to Canada.
Whatever the item is, keep checking in at ebay. Sooner or later someone else will offer the same
thing, - perhaps at a better price.
Ebay is like that. No matter what it is in this world, - if you can possibly imagine it and it's legal,
it will eventually show up for sale somewhere on ebay.
Good luck. 
