If this is a hobby, you report the income as the difference between your income and your cost (zero I assume). You can't deduct any expenses. This is "other income" or "miscellaneous income".
If this is an "ongoing trade or business" (something you do regularly with the intent of making a profit) then you are self-employed as a business. You file a schedule C to report your income and expenses, and pay income tax and self-employment tax on the net profit. Your expenses would mainly be mileage I suspect, assuming you keep records of the mileage you drive around town looking for scrap and taking it to the yard. The IRS expects your to keep accurate business records of your income and expenses, even if you don't get tax paperwork.
I suspect that if you had kept records of your mileage, you probably have no net taxable profit. At 58 cents per mile, you would only have to drive 1200 miles or so around town to zero out the income. But without records -- such as a mileage diary showing the date and mileage of each trip -- you would lose the expense deduction on an audit. In the future, if you document your scrapping trips (looking for items and your trips to the yard) you will probably find that you have little or no taxable profit.
As a practical matter, since there is no tax paperwork, the IRS is unlikely to audit you if you did not report the income.
