https://www.irs.gov/newsroom/form-1099-k-frequently-asked-questions-general

> You should include all fees (e.g., selling fees, payment processing fees, etc.) associated with the sale of your personal items in your basis when computing your gain or loss on the sale. See Publication 551 for additional information. In general, you should adjust your gain or loss on the sale of your property by the amount of expenses and fees paid to facilitate the sale.