I sent the item at my own cost but have not received payment for the item or the courier fees :(

Sorry to hear this Leanda. Unfortunately not everyone is honest or trustworthy and the golden rule is never ship anything until you have received payment. Many of us, myself included, have had to learn this lesson the hard way.
If you haven’t done so already, I would email the buyer telling them that you shipped the item in good faith and now require payment.
If the buyer has not replied to an email asking for payment, I would place neutral blue feedback stating that you shipped the item in good faith and are still waiting for payment. I would also state that unless payment is made within 24 hours you will amend the feedback to negative.
Other than that, there is very little you or Trade Me can really do. I hope the item was of a low value and you are not significantly out of pocket.

Andrea is quite right - this is pretty much your only course of action.

If that fails your choices are down to - walking away and chalk it up to lesson learned, or you are looking at small claims court.

In the past when I have shipped items accidentally before payment, I have got TM involved and they contacted buyer. In most cases they paid. For 1 case they did not pay and TM put their account on hold until paid. To this day 5 years later no more feedbacks on it so account either frozen or abandoned. If you do place feedback on them, dont call them a thief as it is effectively calling them a criminal (but they have not been found to be that in a court) and TM will likely remove it if requested. But you can describe your story in feedback and state you "belive" them to be very dishonest, 

Kia Ora Leanda,

I'm Sorry to hear this has happened also and great advice given above.

There is the option of opening your own dispute report with TM under Seller Protection 

Disputes resolution

Our disputes resolution team aims to resolve disputes between sellers and buyers as quickly as possible. When a seller is unable to resolve a dispute with a buyer, the team will work with both parties to try and help them reach a resolution. This may include applying our Buyer Protection policy where appropriate.

You can raise a dispute report with TM through the "Live Chat" facility.

The 'Live Chat' manned hrs Monday - Friday are 8:am - 6:30pm & Saturday - Sunday 8:30am - 4:pm. However today, Friday 13 July 2023, being a Public holiday for Matariki the manned hrs close at 4:pm.

If you go through the 'Contact Us' link at the very bottom of this, or any, TM page yes you come to a drop down form however if you give the page a few seconds the 'Chat' bubble will come up in blue at the bottom right of the page, there is a slight delay to it appearing at first - All the Best with Outcome here - Cheers.

Cheers Donal, - I didn't see your Comment as researching mid responding here - Pleased to hear you obtained TM Support & Most cases worked out for you - That's Hope for  Leanda Right There! - Cheers 😊

Hi Leanda, sorry to read this. We would definitely want to follow up with that buyer on your behalf. Please do reach out with the trade info and we can lend a hand.