It is better to turnover just under than just over so you dont need to register. As Ebay does not allow you to actually charge more for GST than if you dont have to include it, this means you pay an extra 10% of your profits in tax every quater.

Doing a quaterley BAS does have the advantage that your end of year tax is easier because you have been forced to do most of the bookwork each quater. Helps give you a profit snapshop every quarter, 

The principle of GST with its deduction system is that you are only charged 10% of the value you added to the item (profit margin)