Regarding cars.com.au, there is a lot of credibility that come with it, it just sounds like a big site, a leading site.
But I have to agree with what goldnugget is saying, for anything but the largest venture it is way to broad. It suits someone like a carsales.com.au sized business, yet they own it and haven't done a thing with it aside from setting up a redirect. I'd say the name sucks in many ways despite its huge value. For the smaller web master they'd be way better off developing something specific.
Tim say "cars , someone wants a car so you place good content and you have a winner." I say bulldust that won't make a business or anything close to it. Put up 1000 articles written by your favourite content writer and the sites value will still be no more than the value of the name in my view. Might as well buy a block of land in Toorak and pitch a beach tent on it. Get 30,000 dealers to pay to advertise their inventory on your site and you'll have a winner. Don't worry it will only cost you about $1 billion to create that level of activity. There isn't much of a middle ground in my view, it is not going to have any relevance as a small site.
Most one word EMD's are going to be very difficult to develop. Something like credit.com.au for example - very little to latch onto. The people searching for "credit" will be looking for all sorts of things, so the EMD benefit is going to a questionable keyword. Cars.com.au is a bit like that, who really wants to target "cars"? What are they looking for, to buy cars?, sell cars?, pics of car?, new cars?, used cars?, car dealers in Sydney? There is two advertisers on the term, carsales.com.au & carsguide.com.au, nobody else is big enough or silly enough to bother trying to target this term. I suspect it is one of those keywords where google has priced it to hell because it is so vague & the advertising performance so poor (a bit like "sydney" or any other geo term).
The ideal domain is probably something with massive volume and extremely specific, but those names are uncommon in the extreme, especially one word names, because most single word domains describe an industry or class of products, not something specific. So what to do with great domains like this? Probably sell them.