Lane-ends, You raise a very important point in that the 10% GST you pay the ATO, usually comes straight off the top of your profit, ( assuming you are charging the maximum you can already for your product. ) This can make the difference between remainig competative with other sellers or going under. If your actual profit is 20% of the total sales price you would have to sell twice as much ( $150,000 as opposed to $75,000 )  to make the same profit as before if you start paying 10% ( half of your profit ) in GST. This does not take into account all of the extra costs such as labour, stock on hand, storage costs etc. to handle twice the number of sales. I assume many businesses on ebay would be marking up by more than 20% , but it is still concievable that you would need to sell perhaps $100,000 - $120,000 worth of stock PA to make the same profit as selling $75,000 ex GST. ( not including handling costs on extra sales ). This is a vital piece of information sellers need to get thier head around before launching into the GST system. The maths used here for examples is simplified as things are complicated by input tax credits etc.

             Having said that, I have found in several businesses I have owned over the years that they just seem to take on a life of their own and grow like topsy, so there are times when you need to bite the bullet, take on the GST and let the business growth run.