Normally, I'd be inclined to go with a new one if for no other reason than the battery should be in a little better shape than one that's been cycled. You'll find that even 'factory refurbished' units often don't see a battery swap. However, I wouldn't go with a 730 from the U.S. The chances that one of those will be coming with Aus/NZ maps are nil, and you'd wind up having to buy those on top of the price of the unit. To avoid that, you need to pick a unit up in your own market, so the game swings to the locally procured refurb for a 730. I'd watch the auction, and see if you can't get that unit for AUS$150 or so.
Given that you had a ONE 2nd before, you might also be looking at a 740. The ONE 2nd never had the aux audio out or the FM transmitter that you'll be getting on a 730, and the 740 has the distinct advantage of having current firmware. What kind of a deal can you get locally on a 740? I'm sorry that I'm so unfamiliar with which models TT has been marketing in your region and what the prices look like.