Because a 1099-K will probably be issued, you have to deal with this on your tax return, but the 1099-K by itself does not mean the money is taxable. It depends on what you did to get the money.
You might be selling as a business or a hobby. Since one of the key factors for treating your income as business income is the desire to make a profit, selling old used items for less than their purchase price is not a business for tax purposes. However, if you are buying and reselling for a profit, you have a business. Your taxable income is your sales minus your costs, and one of your costs is the cost for inventory (the items to buy to resell). You need to list your business activity on a schedule C, where you report your sales income, your costs for inventory, your other costs (shipping, listing fees) and so on. You pay income tax on the net profit.
You will have to first determine if you are a hobby or a business, then go from there.
