There is no online facility to appeal a case which has been decided in the sellers favour (ironically there is for a buyer)
However you can call Ebay (Customer Support/ Contact Ebay/ Call us) and take their advice. I have found this very supporting.
At the end of the day, it's one person's word against another and the empty envelope syndrome is always a risk.
Morally, I would have thought that an Ebayer with the better rating might have the edge in a dispute

Another way is to put it out through Twitter, etc, where Ebay may have a 'watcher' to maintain their reputation.
You probably know all this already, but I hope it helps.