As Lyndal says the GST is only payable on eBay fees.
When you are acting as a buyer eBay do not charge you any fee so you are therefore not subject to this new GST.
As the seller you are charged fees by eBay for the use of their services so in turn you ARE subject to the new GST on their fees.
If you are a seller who is registered for GST (ie sales revenue over $AUD75K per snnum) then you will need to account for this GST in your quarterly BAS statements.
If you are not registered for GST then it will not bother you too much but you might wish to have a look into increasing your sale prices marginally to cover it off.
But for a sale of say $10 where the eBay fee would normally be say $1 then 10% on that $1 fee is only 10-cents so not much of an increase really.
Remember the new GST is only applied to the eBay fee component not the full sale price of the item itself.