You can try Trade & Exchange: http://www.trade-exchange.co.nz/
The internet ads are delayed, so if you want to be first in, you have to buy the magazine.
One problem is that most people's idea of what their hardware is worth is completely unrealistic. They think it depreciates by a bit each year like other things, not realising that the NEW price for the same hardware is constantly dropping.
So be sure to check the current retail for anything you buy second-hand - you may need to reset the sellers expectations fairly sharply over the phone.