The IRS will expect you to report the 1099-K in some manner. Let's start from the top:
If you are buying and selling stuff online, it is either a hobby or a business. It is a business if you are engaged in an "ongoing trade or business" and conducting your affairs in a businesslike manner (advertising, seeking new business, trying to make a profit, etc.) You report your business income and expenses on schedule C and the net profit is added to your other income for income tax purposes and you also pay self-employment tax.
Let's assume this is hobby, based on your question. You only have a taxable gain (profit) if you sell the items for more than you paid for them. Most of the time this is not the case for selling used clothing and household items. But, you still need to keep accurate records that include the following information: item description, date you acquired it, how you acquired it, price you paid (or information about the value if it was a gift or inheritance), date sold and selling price. Keep copies of checks or print your PayPal statement and highlight it so you can track each payment to your item listing the sales records. Keep copies of your FB ads and so on. You can estimate or use your best recollection if you don't have the original receipts but you must have some kind of written record, even if it's from memory, that will show the IRS that you are selling used items that you previously purchased.
As long as you sell items for less than you paid, you don't not have a taxable gain. But, you can't deduct a loss. If you sell items for more than you paid, the difference is taxable. As a hobby, you are not allowed to deduct expenses like shipping.
Let's take the situation where you are selling used items as a hobby and most of the items are sold for less than you paid. You don't have to list these items on your tax return, but you do need to keep a spreadsheet or other records. If you sell any items for a gain, you will list those items on the "Sales of assets and other property" section of the Income page, they will go on schedule D. There are three options to deal with the 1099-K.
1. List the 1099-K as income, say it was a hobby, not for profit, not a business. It should go into the program as "other income" and not as a schedule C business. Then, go to the "other uncommon income" section at the bottom of the Income page and enter an item of "other income" that is a negative number to balance the 1099-K. Use a reason like "1099-K offset". That will zero out the income. The IRS may send a letter asking for an explanation. If they do, you will send a letter explaining the situation and a copy of your records that proves that the 1099-K money was not actually taxable income because the proceeds were less than your original costs.
2. Ignore the 1099-K. The IRS may send a letter asking for explanation. Respond as above.
3. Don't enter the 1099-K in Turbotax. Instead, print your return and file by mail. Attach a copy of the 1099-K and a 1 page letter explaining why you did not include it as taxable income. Don't include your proofs, keep them available in case the IRS asks later. You won't e-file in this situation.
