Having an ABN does not automatically imply that you are registered for GST or need to be. These are two different things.

My wife and I operate two ebay stores and each of us has our own ABN.

But we are not registered for GST since neither of us comes even close to the $75K threshold in total annual sales volume yet.

But we are subject to normal income tax and that is why we employ the services of a registered tax consultant/agent.

I would strongly suggest you do the same if you are not very familiar with tax requirements. The ATO will not listen to "I did not know" if they haul you up for a desk audit. You have to know, or at least have a tax agent on board who does know, thats what they get paid for.