My two penneth for what its worth: if you are starting a business on ebay and are serious about it 75k is not a lot of sales and it won't take long to get there ( just over 6k a month in sales). Why not make it easy on yourself and just register for GST from day one. If you are a casual seller just clearing out the wardrobe going on free listings its probably not worth it unless you are selling high value items. Of course there will be exceptions to the rule as always.

Benefits of GST registration:

- No nasty unexpected invoices from the ATO if you go over the 75K in income ( not net profit as wrongly state elsewhere on this thread).

- You can claim a tax input credit on most domestic purchases that incur GST as a legitimate business expense.

- Most data required can easily be retreived from eBays systems if you have sales reports set up.

So if you are worried about being uncompetitive due to having to charge GST bear in mind if you are buying product in Australia to sell if you register for GST you can claim 10% tax input credit on the purchase from the supplier and essentially it's a bit like a 10% discount.  So you list 10% cheaper ergo you are at the same price as all the sellers not charging GST. Also if you register for GST and are importing you can claim back the GST on imports over $1,000 if you register for GST. Of course under $1,000 is GST free (there is still the 5% import duty to pay over $1000).

I didn't register for GST when I started and it did catch up with me ( now all legal) . I wish I had now. So I am a good example of what not to do as non registration probably left me about $500-$1000 out of pocket with ATO fines, extra accountants advice needed etc.