As a US resident you are required to pay tax on your world wide income form whatever source derived unless specifically excluded by law.
Your activity on e-bay is a business because you are turning a profit. You will only be taxable on your income after expenses. However, as you are self employed, the income will also be subject to self employment tax.
You will report your income and expenses on Schedule C on your individual return. I have included links to IRS Publicaation 334 and Schedule C to assist you.