Disregarding the back-story...
Is one bad feedback really that significant? Is the person possibly dangerous?
If the answer to either is 'NO' and you truly in all good conscience KNOW that the item was functional, tell the buyer to go soak his/her head. If the money involved is a small amount and the feedback does matter, give 'em the money. Follow them home, and load their trees full of toilet paper well after dark. Q-tip them. Leave positive feedback worded not very nicely. Steal their girlfriend/boyfriend. You could buy a voodoo doll and MAKE THEM PAY!!!11111
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