Numerous backpackers, who are at the end of their journey, are in Cairns during the local winter months betwenn May-September. During that time the temperatures are still very pleasant (about 25° Celsius). Many travelers are selling their cars and vans, so if you just arrived to Australia and are looking for a car or need a new one, this is the time and place to be.
The cars are quite cheap now. Of course, if you are trying to sell a backpacker car, the situation is not ideal and you should expect that you are not getting that much money out of your car.
The peak in my experience is in early August – there are definitely way more people selling their car than people looking for a new one.
If you are looking for a car in Cairns, you should especially visit the parking lot at the two ends of the lagoon (shown on map) – often cars and vans are marked with “4 sale” sign. Backpackers who sell their cars often leave flyers in hostels as well and of course an ad on www.gumtree.com.au.