Yes you can enter it under Supplies. It does give you 100% write off. It reduces your self employment income by 100%. You only pay tax on the NET profit and you can deduct a Net Loss against your other income.
But Some expenses, such as home office or section 179 depreciation can only be used to reduce your schedule C taxable income to zero, and not to create a loss. Excess deductions for these carry over to the next year. And you have to answer yes to both questions about exclusive and regular use, not just one. The area of your home office must be used regularly and exclusively for business to deduct it.
Or you checked the box on 32b saying Some Investment is Not at Risk.
