You would first enter the 1099-K income and then you would enter the goods sold, sales taxes paid and other business expense in the business expenses section. Once you enter the goods sold and sales taxes as a business expense then this will automatically make the adjustment in the income on your Schedule C so that you won't be taxed on the entire amount listed on the 1099-K.
You can type "cost of goods sold, self-employed business" in the search box and click jump to "cost of goods sold, self-employed business" to located this section and then you can enter this expense.
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(see attached screenshot)
