First contact the seller and ask how to proceed. Ask if you can send pictures of the damage rather than making an expensive return of an unusable (?) item.
He may demand that the item be returned. You should use a service that includes Delivery Confirmation. If the payment was over $250 CDN including shipping, you will need Signature Confirmation of Delivery.
Given the size of the item, a courier may be your best bet. Shop around for price and remember to mark it as Returned Merchandise including a COPY of the original invoice, so your seller doesn't get hit with US duty and tax.
He is not required to refund return shipping.
If he refuses to agree to refund on return, you go to the Resolution Centre and start a Not As Described dispute.
http://resolutioncentre.ebay.ca/