Selling an item as authentic through ebay when it is not is bad enough. If you forge the document as well thoug you open yourself up to a lawsuit by the agency's whose documentation you forged. Plus there is jailtime involved in most states for forging documents, and if it crosses state lines it becomes a federal offense.
So yeah, it wouldn't be too hard to forge the document, but the penalties for getting caught triple as well as the possibility to be sued.
EDIT: There is a reason those big name auction houses etc, keep their reputable name. Because they feverishly protect that name, through all possible legal options.