Q. Isn't the total amount sufficient if not taking expenses etc ?
A. Yes. The IRS will be mostly satisfied that your paying tax on the gross proceeds rather than just the profit. But, there's still the risk that they will want self employment tax in addition.
Q. I don't need to itemize do I ?
A. No, not on Schedule A (personal itemized deductions). But you would deduct expenses and the cost of goods sold (COGS) if you file Schedule C.
Q. What makes the Schedule C mandatory, if you're paying on the entire amount ?
A. If you have a profit seeking business, you are considered self employed and have to pay social security and medicare tax, in addition to income tax. "Self employment tax" is how the self employed pay social security and medicare tax.
So, it depends on what you are selling. If you acquire stuff for the purpose of selling it, you're in business (Self employed). If you're just selling old stuff from around the house, you have nothing to report (unless you sell it for more than you paid for an item)
