Ebay are not the "supplier" rather only an intermediatory.

For AusPost to collect the 10% GST may have been a logistical nightmare, but if indeed AusPost is the point of collection then to add on a $5 security surcharge per item is only minimal extra effort.

It may well be that the Government enforces the GST impost at the point of sale.  That is, they require (for example) Amazon USA to add on 10% on all sales to Australia.  Similar to how they do on some interstate sales within the USA.

Going after the main name sellers would capture a very large % of the sale of new items.  Second hand goods, such as stuff flogged on EBay's overseas sites would not be subect to GST, on the same basis as they are not subject to GST locally.

Will be some items escape the GST net but the cost to capture those extra items may not be worth the effort