Where to buy second hand mountain bike in Sydney?

[theAlainVincent]

I am going very often to Sydney for work and spending many weeks a time... thus weekend rides would be great...
Since Australia is wonderfully rich in trails, I decided to get a cross country entry level bike... something like Specialized Rockhopper 29er or Merida Big Nine...
buying second hand seems a great option in a country so active in outdoor sports... but after trying to select on bike exchange and gumtree, I realised that I don’t know where to find a large listing of second hand such bikes...
Would anyone be able to guide me on the best way to move forward with this second hand idea...
...if nothing possible,I will turn to Clarence street and shop for new...
Hope you can help,
Thanks in advance
Alain