I started an ebay business a few months ago expecting to make about $100 a month in profit and I wasn't going to bother setting up a legit business, ie do taxes on it because I thought it would be about $20 a week. Now I am finding I'm making about $100 a week in profit.

My question is, even making this small amount, I'm still below the threshold and won't need to pay tax as my income is so low. If I want to put this on my tax form, I have to get an Australian Business Number and do quarterly forms (I think, etc) and keep records and it is all such a bother. Will the tax department be alerted if I do nothing and just keep making money on ebay? Or should I go to the extra trouble of setting up an ABN etc...? thx in advance for your help.
Also, a lot of people sell stacks of personal stuff on ebay...when does it become a "business" ...at what volume does it become a business b/c most of us sell stuff on ebay anyway?