If you'd read your user agreement when signing up to ebay (or checked it now you have concerns over GST, etc) you would know that Australian registered users are dealing with eBay International AG, a company based in Bern, Switzerland. You Ebay monthly fee invoice does not include any GST charges as you are dealing with a company in Switzerland which is not required to collect the Australian GST.
As your concerns about fees indicate you're looking at ebay sales from the perspective of a bulk / regular seller / trader rather than a casual seller I suggest you look into the various packages offered by ebay for bulk & business sellers which can offer far better fee structures for you scenarios.
Well we did look at a comparison with a shop that sold 3.5k last month, that is where the two hundred dollar increase occured.
Also we can look at a single item selling at say 10.00 for a casual seller. Compared to when we signed up pre compulsory paypal offerings the total charges are four times what they used to be.
For a person selling an item in an auction starting at 99 cents and finishing at $75.00 the charges are nearly double what they used to be.
For a person selling a loco at say $250 the fees now are close to 40.00 Whereas it was $15.00 pre paypal and $27.00 till now.
For a person selling say a brass loco at say $2500. the fees now will be about $320.00 compared to 80 to 100 in days of old.
So yes the only people better off now are Ebay shareholders.
And for the larger sellers you also need to consider the cost of fraudulent claims by buyers. Having spoken with several sellers there is now a substantial number of false claims made and paypal/ ebay always favour the buyer which results in considerable losses for sellers.
Yes the whole picture is really rosy for sellers.