That's what Schedule D and its accompanying forms are for. Schedule D is most often used for sales of stock and other financial assets, but it can be used for any asset.
You enter a description of the item, the sales amount, the date of sale, the cost basis, and the date you acquired the asset. Cost basis is what YOU paid for the item, or what you have "in" it. If you owned the item less than six months, that's short-term capital gain/loss, which is fully taxable. Otherwise, it's long-term capital gain/loss, which is taxed at a reduced rate.
If you've taken a loss on the sale, that's a reduction to your overall income.
Unless you are audited (rare for those of us who aren't wealthy), the IRS takes you at your word. If you are audited, some record of the purchase and sale transactions will be helpful, even if it's just emails you've saved.
What you DON'T want to do is leave the sale off your tax return. Doing so may land you in the Underreporter Program - not an audit, but you'll get a letter asking you to explain, along with a bill for tax, penalty, and interest should you fail to "explain it away."