There is info on the ATO site for overseas sellers and how they charge the GST.

The way I read it is if eBay uses the simplified system they will only have to charge GST to consumers (anyone not registered for GST).

They do not provide a Tax Invoice and cannot claim Tax credits.

https://www.ato.gov.au/Business/International-tax-for-business/In-detail/Doing-business-in-Australia...