the postage not received, but the buyer paid for the listing stuff already

You could place feedback like I do saying, Thanks for your payment for the items. I have emailed several times with delivery cost but not seen payment or had replies? Can you please check emails/junk folder and reply? Many thanks.

Then wait.

You can also check in case they have given you their phone number and call them, or check them out online to see if you can contact them another way.

If no reply/payment, you can contact Trademe and ask them to contact the buyer.

To avoid this problem next time donaldchoong you need to add your shipping costs to the auction. 

When you have Ping enabled, but no shipping costs you are always going to have this problem. 

Add the shipping costs so buyers can pay everything then and there with Ping.