I very much suspect that some sellers with those high sales figures have been focused on undercutting their competitors, without taking into account that they must pay GST and report their eBay income. They've - in a sense - thumbed their nose at b&m sellers and joined in the hoo-ha about being able to find things online for a cheaper price, but unless they deal fairly with GST and reporting of income to Centrelink as well as to the ATO, they are GOING TO BE CAUGHT OUT.
If there are savings to be made by purchasing online, it must not be at the expense of doing the right thing tax-wise and benefits-wise.
I have to admit I struggle to have sympathy for people who systematically rort the system.
It's different, of course, for sellers who honestly DO sell things from around the house. Collectors who are parting with much-loved items, selling one's own car, clearing out no-longer-needed furniture or clothing - all of that sort of thing is fine. It's not income, and it should be a simple matter to show the ATO that this is the case... no need for an eBayer to panic or worry.
However, when even a cursory glance at the eBayer's completed listings shows that the "selling things from around the house" is simply not credible, I do find myself thinking, "Oh dear - you've been selling like a business, but without accepting the financial and other responsibilities of a business. That is bad, bad, bad."
I agree that where possible, the ATO would probably be wisest to caution the seller concerned, arrange for the money owed to be paid, and give the seller all the information needed for him/her to operate properly. (The ATO website is chock-full of the right information, and an accountant will be able to set the seller straight SPECIFICALLY.)
But where someone's got a history of doing something similar, or has failed to heed previous cautions, or where there is other information that makes it likely the seller was knowingly attempting to defraud on their taxes or to continue to claim benefits to which they were no longer entitled - I am very tempted to say "THROW THE BOOK AT THEM". I have zero sympathy for people selling things that "dropped off the back of a truck", or selling fakes from China (or elsewhere), or selling things they don't have, or selling things while raking in benefits and so on. Deliberate cheats... If they get away with it or can talk their way out of it, I do think it perpetuates a culture of bludging, cheating, deceiving and conning. That can't be good.