It depends on what it looks like. If you can list it on ebay, you can send it to the US, - but permissible weapon-like items are severely restricted. Have a look at the following link, and note the section on Airsoft, replica, or other immitation firearms:
http://pages.ebay.ca/help/policies/firearms-weapons-knives.html
You will see you can only list paintball guns if they "don't look like real guns" and cannot sell them at all if they do "look like real guns and don't have appropriate markings". Even if you think yours will be okay, you may find your listing pulled by ebay. They're sensitive about gun listings, - understandably.
