I sold a used Jet Ski electrical item (ECM) on GumTree. I included the shipping in the selling price. The buyer contacted me saying there is a fault with the item. I was not aware of any fault with the item. The buyer is now asking for a refund. I have a few concerns: 1) How do I know he is testing the item I sold him? 2) How do I ensure he returns the same item I sold him and not the bad ECM he replaced? 3) Do I have to refund him? 4) Can he sue me if I don't refund him under Australian Consumer Law? I have bought things off GumTree that didn't work and just figured that was the chance I took.