It appears the sale of your personal items falls within the Massachusetts "casual and isolated sales" category. Please see: <a rel="nofollow" target="_blank" href="
https://www.mass.gov/guides/sales-and-use-tax">https://www.mass.gov/guides/sales-and-use-tax</a>
If so, you don't have to report the sales since they aren't taxed at the federal or state level. You have two choices:
1) omit the income and if you hear from the IRS or DOR, reply with an explanation, or
2) include the income on Form 1040, line 21, Other Income with two entries, first to report the 1099-K income and second to report the 1099-K as a negative number with a brief statement such as "sale of personal items at a loss."
For the second option, select Income & Expenses > Less Common Income > Miscellaneous Income, 1099-A, 1099-C > Other reportable income. You will enter the income then negative amount separately.
