It's not a requirement that you have an ABN when you start a business. The ATO site has the best information you can get rather than anyone here. Just because you have a shop does not mean you are running a business. It's just a different means of presenting goods you have for sale. You should seek advice on turnover, and consider your line of goods. Collecting and selling, for instance can be seen as a hobby and would need to demonstrate a profit of $20,000 from memory before it is considered a business. Some of this information I have provided, I gained from my accountant ( not for selling purposes just a general accountant). Plese take this information as a general discussion rather than advice. Only you can be up to date on your particular and individual tax responsibilities.