I recently did just that - bought a car interstate (Vic) and brought it back (Tas) on an unregistered vehicle permit.
Personally I'm not keen on buying a car without seeing it, but you have to be prepared to walk away even though you may have travelled a distance to see it - I tried to organise a couple of 'backup' cars to look at if the car i was after turned out to be a dud (thankfully it wasn't!)
For me acquiring the permit was no biggie as the car had both a RWC and rego. which was cancelled when i bought the car and got the permit.
I flew over and drove the car to the ferry terminal and then home as this was the cheapest way for me to get the car! No real hassles from my experience!