You would normally just add your profits from your eBay/online business to your tax return at the end of the year. If your overall earnings (eBay + any other income) are $75,000+, then you need to apply for an ABN.
If you are buying items for the purpose of reselling, then you need to start keeping records for tax purposes.
If you have over $50,000 turnover from your ebay business, you will need to also register for GST. This means that all your listing prices need to include GST, which will need to be reported on a Business Activity Statement, and it also means that you can claim the GST which you pay on expenses associated with your ebay business.
If you cannot afford the services of an accountant, or would prefer not to, the Australian Taxation Office can answer most of your questions over the telephone.