With any interstate car purchase trust is a major factor when it comes to payment, however I have heard of services in which banks can provide gaurentees for a small fee. May be best to check with your local bank regarding this or have the interstae vendor check with theirs.

To ensure the car is as described I would recommend an independent vehicle inspection service in the state in which you are planning to purchase. Only do this if you're extremely interested as can cost anywhere between $150-$300 however will give you a detailed report and evaluation of the vehicle.

Regarding freight I have personally moved a couple cars between Vic & Qld, below are some links I would recommend.

www.movingcars.com.au/
www.carcarrying.com.au/
cevacarcarrying.com.au/

From memory it should cost between $500-$800 Depot to Depot service. Would pay to shop around as some of their rates may have varied since then.

Hope the above info was helpful.