It might have something to do with the fact that Centrelink AND the ATO require people to declare every cent they earn. The penalties are, I believe, quite hefty, so only a mug would think that selling online, in full view of the world, would not attract attention from the relevant authorities.

However, unless times have changed, most Centrelink payments have a threshold, after which it is a 40c in the dollar reduction of benefits and the tax free threshold is about $18k a year, which also allows a significant margin over any benefits. Especially disability pensions. They have around $70 per week before benifits are reduced, on the above lines, and are exempt from income tax as far as the threshold is concerned.