There were two old procedures that could be used.
a. Leave the 1099-K off, file by mail, and attach a written explanation.
b. Include the 1099-K as "other income", then create a negative 1099-K to remove any part that was non-taxable (like items sold for less than their cost).
We don't know yet if the IRS will create a new procedure since so many more people than before will be getting a 1099-K.
You need to keep records in case the IRS sends you a letter asking for more details. You should have a spreadsheet that lists all the items you sold and includes the date purchased, price paid, date sold, selling price, and description. If you don't have records of the date purchased and price paid, make your best guess and hope the IRS doesn't ask for details.
