It pays to be upfront with the tax office with income. However as stated in the article, people selling off unwanted household items don't need to declare and shouldn't worry.

Those folks who are selling as a form of income, like myself, need to keep good records. I've been selling seriously for around 5-6 years and declaring my income over that time - and for the first time last tax year I  had to pay a bit extra tax. That's good - it means I am operating a successful business that is making a profit. Not a huge profit, but enough that I can share it with other Australians who need it. Isn't that what paying tax is supposed to be all about? Paying for schools, health, roads etc. Without it we'd be in a sorry state.