GST and Fees

Basically, all the money you pay to Trademe for goods and service includes a GST component, whether it be for listing, success fees, other services, or goods like Book a Courier. You can get a summary of the GST you have paid, on your monthly GST invoice/receipt from Trademe.

https://www.trademe.co.nz/MyTradeMe/AccountStatement.aspx?gst=true

and if you accept Ping payments, there is GST included in the fees for that too which are detailed on a separate GST invoice/receipt monthly.

https://www.trademe.co.nz/MyTradeMe/Ping/GstReceipts.aspx

so does this mean that if I sell something with for $20 + $10 postage = $30, I pay the TM fee which includes the GST on the $10 postage as well as the GST on the $20 sale fee.  I then book a Courier for $10 which includes the GST on that so I am really paying two lots of GST on the postage

No, you only pay GST in the success fee part, not the total sale price.

So, if your sale is $20 + $10 delivery total $30, your success fee is approx 10% ie $3.00 and that $3 fee includes a GST component of $0.40

If you book a courier and you pay Trademe $10 for the courier, that $10 includes GST of $1.31