Personal items are taxable if you sold them for more than you paid for them. The taxable amount is the difference (the gain).
If you do this as a job (eBay, etc.) then you will need a schedule C for self-employment.
If you do it occasionally, you can report the amounts as capital gains under "income from sales of stocks and other assets." You will need to know the purchase date, purchase price, selling date and selling price.
