Hey Jono,
Looks good, nice and clean.
I would work on your value proposition in the header, make your tag line something like 'Australia's #1 Place to Buy & Sell Car Parts Online'
I reckon you could re-jig the positioning of the elements on the home page too..
[join now] (full length)
[featured] [categories]
[tabs] [top ads]
You could then expand the Join Now box to double as a call to action to post an ad.
Keep up the good work!
Cheers,
Chris