Buying a car interstate can be a bit more challenging than buying a car in the same state.
Firstly, make sure you ask the seller all the relevant questions you have about the car and emphasize that you will be travelling interstate and that it is important for them to tell you everything about the car, any damage and any issues before you fly out there. You should understand the possibility of losing any travelling fees if you don't end up buying the vehicle.
Secondly, you will need to ask them whether they will keep the car registered so you are able to drive it back to your home state, as they have every right to cancel the registration, and as they cannot actually transfer the car in to your name using official state transport documents.
Thirdly, make sure your insurance company will cover you in an interstate registered vehicle.
Fourthly, as for payment, Bank Cheque would probably be the most appropriate method of payment as you don't have to carry cash for a long period of time, and the seller can be confident the funds are locked in as opposed to transferring the funds online which can decline.
Need further help? Simply give your local transport authority (VIC ROADS, TMR, Service NSW) a call for more information about registering a car interstate.