Sold a car, now I'm being threatened with being sued for 5000 dollars?

Buyer bought the car on trademe using a buy now function witout having seen itfirst

I delieveredit to her and said it may need a few things to get warrant but shouldn't be too much - my belief at the time. auction said it was mechanically sound which I believed. Buyer was happy after a test drive. A few days later received a text saying that the car needed repairs to pass a warrant she asked for a 100 dollar refund to pay towards this, which I willingly did - as I knew the car had some minor faults and I felt sorry for her(as a student myself, I know how hard having to pay for repairs can be). This was also when she finally gave me her address, and I transferred the ownership of the car to her. A week later I received a call from her saying that the car would cost over a grand to fix,and demanding I give her a full refund - impossible, as I no longer had her money. The next day she texted me saying she had contacted a lawyer and had a watertight case - either i should pay up now or be sued 5000 dollars!! From what I've read at the most I would have to go to disputes tribunal instead of the district court. Can any one help out