The IRS has received a copy of a form 1099-B, in your name and social security number, for the sale of stock or mutual funds. The gross amount was $13,000. The $13,000 is not taxable income. Only the capital gain, or loss, (the difference between what your sold the stock for and what you paid for it) is reportable income. However, until you report the sale on form 8949 and schedule D, showing your gain or loss; the IRS will treat the whole $13K as income.
Just got sent a CP2000 for the same issue. The IRS misclassified it as on my 1040 and the scary letter caused a freakout at my house. 1) it's investment income so it is capital gains not personal income and 2) The $87k "income" was for the sell price, not the realized gain/loss which was < $2k.
@harpermaddox - If you failed to enter the cost basis when the capital gain was reported then the IRS assumes it was zero and bills you for the entire amount of the sale, not just the gain. Answer the letter with documentation to show the actual gain or send an amended return if they requested that.