Hi, we purchased 2nd hand car from a private seller. He assured us the car had no issues and mentioned that we would need to only change transmission oil in about 6-12 months but nothing major as the car was serviced recently and no issues found. Unfortunately, after driving the car for a few hours a fault light came on and the car showed issues with transmission. I contacted the seller following day and asked him about his mechanic's contact details (as he promised to pass this onto us) but he avoided it and just said that there are no major issues. We drove this car two more times and it became evident the car had pre existing issue and the seller sold us the car knowing it was unsafe and that this problem existed. We eventually managed to get through to his mechanic's and he confirmed that this was know to the seller and this car needs complete transmission to be rebuilt at the possible cost of $8000. We called the seller and he agreed to return the money but has created lots of issues in the process and has now a week later requested recall of the payment to my bank and is threatening to take us to fair trading / court. We have proof that he knew the issue was pre existing as we had his mechanic's on the speaker the day we returned the car to him just to confirm that we have not made this story up and that all parties are aware of the truth. My question is, can we just ignore his threats (as he is now making a lot more issues) and do I have any obligation to any requests from him in regards to fair trading or similar? Thank you for your assistance.