Is it possible for a canadian to sue an american ebay for copyright infringement?

Ebay is not liable for what sellers and buyers do, only in some aspects but not in copyrighted material. If your picture was not protected by a copyright or internet service that does that, any picture on the internet is fair game. The buyer copied your picture but he may have found it on Google images also, so you should check there. If the photo was not copyrighted you cannot sue the buyer, no matter what country you are in. It is difficult to sue someone in another country anyway becaue laws are different and you have to sue in their jurisdiction.