Since domain name transactions are often international by nature and there is some risk of buyers not receiving their purchased domain names and an equal risk of sellers not receiving cleared funds, third party escrow services are fairly common in the domain name trading world. The escrow service providers act as secure intermediaries in the transaction.
For domain sales where both the buyer and the seller are based in New Zealand, TradeMe's SafeTraderTM escrow service can be used. We are aware of domain transactions that have been performed using SafeTrader, though it is really geared up for transactions of goods that require actual shipping.
Internationally, the market leader in domain escrow transactions appears to be the german firm, Sedo GmbH. They provide a secure escrow service for domain names and take a commission of 3% of the sales price (subject to change) for deals that were not initiated from their listed domain sales pages. See their website for more details. Note however that they have a rather high minimum commission charge for .nz domains.