What Glenn says is correct, however there are potential 'gotchas' even if you go over there to see/buy it in person.
For instance, you are supposed to own the car before you apply for import approval (they want to see proof of ownership), so you would either have to stay in the US until approval was granted (can be 3-4 weeks) or otherwise pay for secure storage of the vehicle while you wait. Leaving the car with the seller is an option, but then you're back in pretty much the same situation as if you'd bought the car online from Oz. Plus as you pointed out, what are the chances of the perfect car just happening to come up for sale at the time you're over there?
And what happens if import approval isn't granted for some reason? I don't think there's many people who would be willing to cancel the sale and refund your money 3-4 weeks down the track. The whole process is pretty dodgy IMHO regardless of whether or not you can see the car in person before buying. While there are quite a few seasoned experts on here that could be relied upon to be able to spot a polished up rust bucket 99 times out of 100, you'd need to be pretty sure of yourself to take on that responsibility.
I reckon to give yourself the best chance of not ending up with a horror story, you need the following things at a minimum:
- A seller with whom you can establish a rapport
- A professional inspection service you can trust
- Good negotiating skills
- Attention to detail
- A reliable shipping company with a focus on customer service
- Being prepared to pay a little more for the right car
Keep in mind that vehicle titles are very important in the US and they are generally a requirement for export. Some states do not issue titles for vehicles over a certain age but a good shipping agent can advise you what other arrangements can be made in those cases. If the car does have a title, it must be 'clear' which means there is no security/finance interest ('lien' in US speak) in force. If there is a lien showing on the title, the seller must provide you with written proof from the lien holder that it has been discharged.
Also factor in the exchange rate and shipping/import costs when looking at US cars. Something that sounds very reasonable in US dollars will quickly blow out to close to double that figure in Aussie dollars once you pay for trucking, ocean freight, marine insurance, customs charges, GST, engineer cert, headlights, seatbelts, etc. etc.
In spite of all that, it can be a very rewarding and satisfying experience to import your very own 'stang rather than just pick up one that's already here like buying any other used car. I have done two so far (and looking at a third) and while there have been a few surprises and ups and downs along the way, I'm happy to have taken the plunge.
Good hunting!