EMD (exact match domains) are not as good as they used to be since Google clamped down on people opting to use keyword rich domains over well thought up business name and brands. Google wants individuals and businesses to come up with good branding, business names and just good sounding websites.
You should opt to use something like:
businessname.com/cars-for-sale-in-melbourne
A couple of links to read about the Google EMD Update
Early Reports
While some EMDS websites have been hit harder than overs some still have the same rankings and I would imagine this is because of site age, authority, back links. Like Google panda and penguin updates we never know what to expect to see next. And because of this I imagine we will receive many more EMD updates.
I'm not implying you can't rank EMD's
This isn't to say you can't rank a EMD but the keywords in them no longer hold as much weight as they used to.. in this effect your better of just using a well thought out business/brand name while using rich keywords in the URL's additionally many believe that its actually easier to rank deep links rather than the route domain in anycase because its more natural in terms of backlinking to deeper links with more resourceful information than the front page but this is another debate.