It's a disgusting system and it's painfully obvious that eBay have set it up so that most small sellers will eventually fail and there's very little that you can do to protect yourself from rabid buyers (or in your case, sellers who should know better) who, for whatever reason, feel they have an axe to grind.

Needless to say, when contacted eBay weren't the least bit interested despite evidence that showed the buyer was clearly being malicious, so I feel it's only a matter of time before I'll be given my marching orders, not that I particularly care any more, because I'd pretty much planned on giving eBay the big flick anyway.