This is a situation ... in the past you were still required to report this income but now with the 1099-K forms being issued you are forced to report the income. On the Sch C you can also report the expenses along with the income. If you cannot figure how much you originally paid for the sold items the IRS assumes the cost is zero so a guess is better than nothing.
If you are selling personal items for a loss you cannot take the loss so the cost of goods sold would be limited to the income on the sale.
