The price difference you get from buying and selling your stuff should be treated as your gain or loss.
Because when buying this stuff, you had paid the tax. So if you sold it less or even, you don't need to claim it as income. For example,
you bought a car two years ago with 20,000 including tax and this year you sold it at 16,000. This amount of money is post-taxed and you don't need to claim as your income.