Well personally I dont think Trademe should be held responsible, as I mean with any rentals no money should be handed over till a contract is signed etc... Once you go outside the trade its not Trademes responsibility, And it doesn't sound like the proper process was followed at all, if it was there would be a paper trail and over seas landlords usually go through a family member living here or a reputable real estate company. As for the IP address it could of been set up from an NZ computer or such and the money sent to the UK as they didn't state anything in the news article about the IP, and even Trade and Exchange warn people against using Western Union as payment as its usually a scam.
IP address alerts and address verification is not going to stop people getting scammed.
TradeMe need to increase their security to make it more difficult for the scammers. Address verified would be a start.