I need information on Sellers Rights.?

I sold a vacuum cleaner recently on ebay that was in excellent working condition. There was no faults with the item while in my possession. On the day it was sold i used it to clean my living room and cleaned it to the best of my ability before collection. The buyer refused the offer to try it out before taking it home, use it, left positive feedback saying it was in excellent working conditions. Now, the buyer has opened a case against me saying i sold it damaged and is demanding a refund. I know these allegations are false but how can i protect myself? Surely buyers can not leave positive feedback saying it is working after using it then claim it is faulty... please help x