Don't make a big deal out of it. That only makes things worse. See the "Streisand Effect" or the current online battle between Amy's Bakery and the Kitchen Nightmare show with Gordon Ramsay. Both are perfect examples of how attacking negative feedback can blow up in your face.
You can give your side of the story on the ebay site - just be nice and calm about - no accusations, etc.
Or you can work something out with the buyer. Maybe give him his shipping money back in return for him posting something positive instead.
I don't recommend any legal action.
In these situations, it is usually easiest and cheapest to try to work something out with the person so he removes the negative feedback. I would ask him if you send him a check for the shipping if that would resolve this.
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