When you enter the Form 1099-K, you should choose Personal item sales on the screen that says Which type of income is your 1099-K?:
On the screen that says Let's get the info from your 1099-K, enter the sale amount that shouldn't be included in your income, which will be $0 if you sold all of your personal items at a loss:
The sale will appear on your tax return but will not be included in your taxable income: