I sell a few things on ebay from time to time and I agree it can be a lottery when a buyer with unrealistic expectations raises an issue, whether you are in the right or not. Not long ago I sold a marble coffee table for my parents and the buyer came to Poole to collect from over Hastings way, even hired a van too. He was nice as pie during the collection but once home he raised a dispute saying it was not real marble, because the underside hadn't been polished he thought it was concrete! Anyway I scanned the original receipt which clearly stated it was marble and to my surprise I won the case, so ebay can and do side with sellers.
It sounds like you did nothing wrong and as the cable wasn't a part of the listing I would hope ebay will do the right thing and side with you on this.