This could be a hobby or business, depending on your motive and how you carry out your activities. Here are a couple of tip sheets.
https://www.irs.gov/newsroom/hobby-or-business-heres-what-to-know-about-that-side-hustle
https://www.irs.gov/newsroom/know-the-difference-between-a-hobby-and-a-business
If it is a hobby, you report the income as miscellaneous other income. You can't deduct any expenses. You pay income tax on the income.
If it is a business, you report it as self-employment income on schedule C. You report your gross income, and you can deduct business expenses if you have adequate records (that might be mileage on a truck used to collect and haul the scrap to the yard, for example). You need reliable records of your expenses in sufficient detail to prove, if audited, that they were business expenses. You pay income tax and self-employment tax on your net profit (gross income minus expenses). If you pay to buy scrap from people, that can be one of your expenses, but normally I would expect you don't have any actual costs for your materials, just maybe vehicle expenses.
