If you have deducted the cost of goods as a business expense, then any amount you receive for selling them is taxable income.
By deducting your cost, you reduced your "basis" in the items to zero. (One way of describing basis is that it is the amount of already-taxed dollars you have invested in something. By deducting the cost, you now have zero after-tax dollars invested in the items.) Any item you sell for more than it's basis is either ordinary income, or capital gains, depending on how long you owned the item and why you originally purchased it. In the case of selling items that were purchased as inventory, the income is ordinary taxable income.
If you never deducted the cost as business expense, then your basis is the price you paid. Selling personal items never creates a loss you can deduct, but you don't owe tax unless you sell for more than the basis.
