eBay and PayPal fees have no GST, so are not claimable until tax time, against your final taxable income for the year.

However if you are registered for GST you need to charge your customers GST and pay it to the ATO every 3 months.

Any expenses that have GST attached that you have paid out within that 3 months is claimable against whatever GST you are due to pay.

This includes any receipts collected for postage paid out by you for that 3 months.

That is how I have been working since 2000 and have had a tax record keeping check (not an audit).

Seemed to be OK with the ATO.