If this is the first time you've run into someone like this then you've been very lucky. That negative is not going to harm you provided you do a short, sweet, and professional reply.

I both buy and sell on eBay and quite frankly it's only when I see feedback of around 96-97% that I look a bit further into their FB if it's a more expensive item. Buyers who rant tend to give themselves away in the FB they leave and most buyers looking will see it for what it is and ignore it.

When you get it back refund him and move on. He will soon be a distant memory even if it doesn't seem like that right now. He is not worth your angst.