Also try good ol' trademe. If you join the AA, you can get them to check the car out for you and give you a thorough report of any work that would need doing - I think it costs about $130 -ish. You probably have to give them a couple of days notice. Worth doing if (like me) you know nothing about cars, or if you are going to take the risk of buying unseen.
I actually reckon cars might be a bit cheaper down south...there seems to be a bit of an Auckland premium on most stuff...but I suppose you might not get as much choice.
Lil.