The misconception that trips up more stock investors than any other is simple: that tax is only owed when money leaves the brokerage account.
It isn’t. The IRS taxes realized gains — the moment you sell a security for more than you paid — not withdrawals. If you sell Apple stock for a $6,000 gain and immediately reinvest every dollar into Amazon, you owe tax on the $6,000 gain in the year you sold. The fact that the money never left your brokerage account is irrelevant to the IRS.
Most experienced investors know this in the abstract. But in practice — when trading is frequent, when positions are small, when the account generates automatic dividend reinvestments or tax-loss harvesting — it’s easy to lose track of which transactions occurred and what they generated. That’s where IRS notices come from. Not fraud, not complex schemes. Just the gap between what a brokerage’s 1099-B reported to the IRS and what appeared on the investor’s return.
The first thing the IRS sees: your 1099-B
Every brokerage that processes securities transactions sends a Form 1099-B to both you and the IRS at year-end. It lists every sale, the proceeds received, the cost basis, and whether the gain or loss is short-term or long-term. The IRS’s matching system compares that form against your Schedule D and Form 8949.
When the numbers match, the return passes through without any human involvement. When they don’t — when a transaction appears on the 1099 but not on the return, or when the proceeds differ from what was reported — the system generates a CP2000 notice proposing additional tax.
This is the origin of most stock-related IRS notices. Not an agent reviewing your portfolio. An automated comparison between a form the IRS received and a return you filed. Avoiding most stock-related IRS issues comes down to making those two sources match.
Holding period: the factor worth most attention
The difference between a short-term and long-term gain can mean thousands of dollars in tax on the same profit — and it turns on a single variable: whether you held the asset for more than one year before selling.
Short-term gains are taxed at ordinary income rates. Long-term gains receive preferential rates of 0%, 15%, or 20% depending on your total income. For an investor in the 22% bracket, selling a position after 11 months versus 13 months on a $15,000 gain is the difference between paying $3,300 and paying $2,250. The asset is identical. The profit is identical. The tax treatment is entirely different.
Active traders face a compounding version of this problem: frequent short-term gains stack on top of ordinary income and can push total taxable income into higher brackets. An investor who earns $70,000 in wages and generates $40,000 in short-term trading gains may find themselves paying 24% on income they expected to be taxed at 22%.
Cost basis: where reporting errors actually happen
The capital gain formula is straightforward — selling price minus cost basis equals gain. But cost basis is more complicated in practice than it appears.
Shares acquired through dividend reinvestment plans have a different cost basis than shares purchased directly. Stock splits change the per-share basis without changing the total basis. Corporate mergers and spinoffs require basis adjustments. Shares transferred from one brokerage to another may arrive with incomplete or missing basis information — particularly for older positions held before 2011, when brokerages weren’t required to report basis to the IRS.
When basis is missing from the 1099-B, the IRS may see the full proceeds as a taxable gain unless you provide the correct basis through your return. This is one of the most consistent sources of investment-related CP2000 notices: the IRS calculates tax on gross proceeds because basis wasn’t reported, the investor pays without checking, and overpays by the tax on the entire sale amount rather than just the gain.
Before filing, compare your 1099-B against your records for any position where the basis field shows “not reported” or a different figure than you expect.
The wash sale rule and where it creates problems
The wash sale rule disallows a capital loss when you sell a security at a loss and repurchase the same or substantially identical security within 30 days before or after the sale. The loss isn’t permanently gone — it’s deferred and added to the cost basis of the replacement shares — but it can’t be claimed in the current year.
Most brokerages track wash sales automatically within a single account, and those adjustments appear on the 1099-B. The problem arises in situations the brokerage can’t fully see: if you sell at a loss in a taxable account and repurchase the same security in an IRA within the wash sale window, the IRS disallows the loss — but the brokerage may not show the adjustment because it can’t see activity in the IRA. The investor claims a loss they’re not entitled to, and the return is incorrect.
The same issue occurs with transactions across multiple taxable accounts, joint accounts, and spousal accounts. Wash sale violations across accounts require manual tracking that brokerage forms won’t always catch.
Dividends — taxable even when reinvested
Dividend reinvestment plans are convenient, but they create a tax obligation that many investors underestimate. Dividends are taxable in the year received regardless of whether they’re automatically reinvested into additional shares. If your account generated $2,400 in dividends that were automatically reinvested, you owe tax on $2,400 even though no cash landed in your checking account.
Qualified dividends — paid by U.S. corporations and most foreign corporations on shares held long enough — are taxed at the preferential long-term capital gains rates. Ordinary dividends are taxed at regular income rates. The 1099-DIV breaks these out clearly, and the IRS matches reported dividend income against your return. Dividend mismatches are among the easiest to prevent: enter the figures exactly as shown on the form.
Reinvested dividends also increase your cost basis in the shares purchased — which is important to track for when you eventually sell those shares. Each reinvestment creates a new lot with its own basis and acquisition date.
What to do when you receive a stock-related IRS notice
The most common stock-related notice is a CP2000 proposing additional tax based on a transaction that appears on the brokerage’s 1099-B but not on your return. Before responding, pull your brokerage records and identify exactly which transaction is in question.
In many cases the issue is a missing basis rather than unreported income — the IRS calculated tax on the full proceeds because no basis was shown, but you actually had a cost basis that significantly reduces the gain. Responding with the correct basis often reduces the proposed adjustment substantially.
If the transaction was genuinely omitted, acknowledge it, provide your records, and pay or arrange payment for whatever is owed. Don’t ignore the notice — automatic adjustments that become final without a response are harder to reverse and accrue interest from the original due date.
Frequently asked questions
Do I owe tax when I sell a stock at a loss? No — losses don’t create tax liability. They create capital losses that offset capital gains and, within limits, ordinary income.
What happens if my brokerage reports the wrong cost basis? You can correct it on Form 8949 by entering the actual basis and including an explanation. The figure your brokerage reported to the IRS doesn’t override the correct figure — you just need to reconcile it clearly on your return.
Are dividends from index funds taxed differently than individual stock dividends? The same rules apply — qualified dividends at preferential rates, ordinary dividends at ordinary income rates. The classification of dividends from index funds depends on the underlying holdings.
Does tax-loss harvesting create IRS problems? Only if the wash sale rule is violated or the transactions aren’t properly reported. When done correctly with attention to the 30-day window and multi-account coordination, it’s a legitimate and commonly used strategy.