Anything you do to make money as an "ongoing trade or business" is self-employment income. If you did this regularly, with an intent to earn a profit, you are self-employed and are running a business.
You need to use Turbotax Self-Employed version if using Turbotax online. You are expected to have kept accurate records of your income and expenses, and report all these even if you don't get tax paperwork. It gets reported on Schedule C which is attached to your regular tax return. Your gross sales are your gross income. Your expenses include inventory (what you paid for the cards), your listing fees, credit card processing fees, and shipping costs. If you have a net profit, that income flows to your regular return and is subject to income tax and 15% self-employment tax.
If you are buying and reselling inventory, this is fairly straightforward. If you are selling old items that you bought a long time ago, you will have to estimate your original cost. If you sell for less than your original cost, then the proceeds are not taxable, but if audited, you will have to be able to prove your cost. If you sell for more than your original cost, only the increased value is taxable income, but again you have to prove your cost. If audited, and you can't prove the cost, the IRS can set the cost to zero and make all the proceeds taxable.
The other way to handle this is as a hobby. If a hobby, your taxable income is the difference between your selling price and your cost, and you pay income tax but don't pay self-employment tax, but you can't deduct other expenses like credit card fees or shipping.
The difference between self-employment and a hobby is mainly regularity and intent to earn a profit.
