Are there any discounts on seller fees as you get powerseller status and move up the levels?

yes, highly illegal if you are running a business and don't declare income. I believe the ATO is planning a crackdown in ebay business as they have identified it as an area of great...

Just be aware if you claim a tax deduction for part of your mortgages (eg if you use 10% of your home to run your business you can claim 10% of the interest on mortgage as a tax...