You should always keep your own accurate records of business income and expenses. If you are filing a schedule C for self-employment because you sell at a profit, you will enter your gross income, your costs for the items, and your other expenses (shipping, sales tax, etc.) and pay income tax on the net profit.
If you are only selling used items for less than you paid, you don't have taxable income and it doesn't need to be reported if you don't get a 1099.
If you are doing something else, we might need more info.
