Ebay would be streets ahead of anything, but now The Trading Post, a very popular form of online, and print advertising in Australia has now introduced an online auction format.
Huge company behind the enterprise, and I have no doubt it will fast become very popular for advertisers and viewers alike.
There is also a site called Oztion, they claim to be the second biggest behind ebay, but daylight is actually second. No doubt in my mind that The trading Post format will fast overall Oztion to become the second most popular behind ebay.
To be honest, they are a powerful enough company to give ebay a real shake if they get serious with the format. I notice that the advertising campaign has started here in Brisbane, and it will no doubt be coast to coast.
So for my money, ebay is the one, but Trading Post would also be on my marketing list as well. What those two won't cover in this country probably isn't worth covering.
If you want to extend to NZ, I would suggest Trade Me.