I think you are confusing GST and taxable income.
You do not need to collect GST until your turnover approaches $75,000 per financial year, which will be hard to do until your store ramps up and you can list more.
However, you have will have to pay income tax on your profits as you are running a store and it is definitely a business. If you were selling 2nd hand items from around the house then you wouldn't need to declare your income, but you are selling new items with the sole intention to make a profit, so you will need to pay tax.
Buy a starter motor for $50, sell it for $115. After removing postage fees, eBay fees, PayPal fees, you might be making $42. This will be the amount you pay tax on, at your marginal rate, maybe 30 cents in the dollar or more, depending if you have other employment.
Of course there will be other part deductions you can use to help reduce paying tax on your $42 profit, such as part of your internet bill, part electricity and rent/mortgage payments if you set up an area of your house to conduct your business. But these costs have to be realistic and worked out with a detailed and professional approach.
The thing to remember is to keep records of everything you buy and sell, as well as your expenses. You will need to be able to prove everything if you have an audit, which is unlikely if your sales are small, but may probably happened as you become a power seller!
