10%?

You pay tax on every cent once you get past $18k pa. Total taqxable income. Not just eBay sales.

eBay fees are 10% or less; Paypal fees are 30c + 2.6%.

The tax you pay is on your taxable income. Nothing to do with eBay or Paypal.

Are you sure you're ready to operate a business? You don't seem to have any idea.