I sold my ex-partner a secondhand car on Gumtree as she is not aware of online selling as a private sale last month September 2016. We asked buyer to get the mechanic and get the car checked before she buys as my partner was driving everyday and she didn't had any problems (2003 Model Citrion) and after the sale about 5 days later, the buyer went to her mechanic and he said there is a problem in the engine and need to get fixed which will cost $2000. We sold the car for $1700 with 3 months Rego on it. And from that day the buyer is harassing over phone, text messages etc and recently we received a letter from buyer demanding a refund. So does the private sale also need to be covered under warranty as we clearly mentioned the buyer to get the mechanic and check the car but they haven't and had a test drive and paid the money. So what is my position now under Australian Consumer Law? Have we committed any crime here and what are our rights?