"If these are personal items and you sell them for less than the purchase price, the sale and income received is not reported on a tax return."
While this is strictly true, it really doesn't cover what you need to do. You need to keep accurate records of your sales, including a listing of each item--a description, the date you acquired the item, how you acquired it, the cost or cost basis*, the date sold, the selling price, and any fees.
*Cost basis is what you paid for the item. If it was a gift, the cost basis is what the giver paid. If it was inherited, the cost basis is the fair market value on the date the previous owner died. If you don't have records, you should make the best estimate you can, including writing down as much detail as you can remember. The more detail you can remember, the more likely it is that an auditor would accept your explanation.
If you sell used items for more than their cost, you have taxable income, but if you sell personal items for less than their cost, you can't deduct any losses. However, it is very important to keep these records, because you will likely get a 1099-K from the payment processor showing how much money you received. While you will claim on your tax return that this money is not taxable income because it only represents items sold for less than their cost, the IRS may (and probably will) ask you to provide more proof.
