If audited, you have the obligation to prove the sales proceeds are not taxable. There is a Tax Court case involving someone in your situation, who claimed she only sold used items for less than the original cost, but had no records. She was audited, the auditor determined she owed full income tax, she took it to Tax Court, and lost again. The Court ruled she had an obligation to keep some kind of business records to support the claim that these were items sold at a loss, and without those records, the IRS was not obligated to believe her story. (The Court also noted that she should have known better than most since she herself was an IRS employee.) She was assessed back taxes, interest, and an accuracy penalty.
You should have, at a minimum, a sales record that lists the items, with a reasonably specific description, the date acquired, how acquired (gift, purchased, inherited), the date sold, and the selling price. If you don't have records, you can use estimates, but note them as estimates, and add any other notes or explanatory details that would help to show the IRS that your memory is reliable. You might even be able to find historical prices for the same or similar objects in something like old Sears catalogs online. The more thorough and complete your records are, the more likely an auditor would be willing to overlook any small gaps. You might use a notebook, spreadsheet, or accounting program.
The key is that the record should be contemporaneous -- that is, the record should be made close enough in time to the sales that your memory would be considered reliable. Even if you can't prove the purchase date with a receipt, your recollection made at the time of sale will be more acceptable than a spreadsheet you make up the night before your audit 3 years from now.
Then, any sales for less than your cost are not deductible, and any sales for more than your cost are taxable income.
If you are not engaged in an "ongoing trade or business" with a profit motive, then this is hobby income. You can't deduct expenses like listing fees, credit card fees, or shipping. You owe tax on the total sales gains from items sold for more than cost. If you get a 1099-K for more than that amount, there is a way in Turbotax to make an adjustment. When the IRS sees that adjustment, they may send a letter asking for more details so they can verify your story. Even though you have done $18K of gross sales, if that was really all below your original cost, then you don't have a profit motive, and this is hobby or "other miscellaneous" income.
If you believe you are engaged in an ongoing trade or business for profit, you can report the activity on schedule C. You still can't take a loss for items sold for less than cost (since these are still ultimately personal items and not business inventory) but you can deduct selling costs (shipping, etc.) from the sales proceeds of items that you did sell for a profit. Your net profit will be subject to income tax and self-employment tax. You can enter the 1099-K as part of your schedule C and then make an adjustment to account for the difference between gross sales proceeds and taxable proceeds (excluding the items sold at a loss).
