no matter what figure ebay report to the tax office, if you are a seller you would need to declare all your income to the ATO

that includes wages if you have a job

all sales and the income you have from charging your buyers postage.

you offset that with the cost of your item / ebays & paypal fees and the cost of your postage you buy from the PO as well as the cost of your packing materials etc.

thats it pretty simply put, your accountant will sort out the finer details for you but yes

postage received is an income

postage and packing purchased is an expenditure