Investment-related CP2000 notices – particularly those arising from Form 1099-B stock sale reporting – are among the most common and most frequently misunderstood. Many taxpayers receive these notices not because they failed to report a sale, but because their broker did not report its cost basis to the IRS.

This causes the IRS to propose tax on the full sale proceeds rather than on the actual gain. Understanding exactly what the IRS is claiming, and how to document the correct basis, is the key to resolving this type of notice. Check a full range of CP2000 notice scenarios here. 

Why Stock Sales Trigger CP2000 Notices

Brokers are required to report stock sale proceeds to the IRS on Form 1099-B. Since 2012, brokers have also been required to report the cost basis and acquisition date for most “covered” securities – shares purchased on or after January 1, 2012. For “noncovered” securities – shares purchased before that date, or shares acquired through certain transactions that did not trigger basis tracking – brokers may report only the proceeds without basis.

When basis is not reported on the 1099-B filed with the IRS, the IRS matching system has no cost basis information. If the taxpayer reported the gain correctly on Schedule D using the actual basis, the IRS may still generate a CP2000 because it cannot independently verify the basis. The IRS proposes tax on the full gross proceeds, treating the entire sale amount as a taxable gain.

Other scenarios that trigger stock-related CP2000 notices include:

• The taxpayer did not report the sale at all on Schedule D
• The taxpayer reported the sale but used an incorrect basis figure
• The 1099-B contains an error in the proceeds, acquisition date, or basis amount
• Wash sale adjustments were not applied correctly, affecting the reportable gain or loss

Step 1: Compare the 1099-B to Your Schedule D

Locate the Form 1099-B from the relevant brokerage for the tax year in question, and compare it to what you reported on Schedule D (Capital Gains and Losses). For each sale listed in the CP2000:

• Confirm the proceeds figure matches what you reported
• Identify whether you reported a cost basis, and what that basis was
• Confirm whether the holding period (short-term or long-term) was correctly classified
• Check whether any wash sale adjustments apply

If you did not report the sale on Schedule D, the IRS’s position is likely correct and an agreement response is appropriate. If you reported the sale with a basis that differs from what the IRS has on file, that is the discrepancy to address.

Step 2: Establish Your Cost Basis

The most effective way to dispute a CP2000 for stock sales is to document your actual cost basis. Sources for cost basis documentation include:

• Trade confirmation letters from the brokerage at the time of purchase, showing the purchase date, number of shares, and price per share
• Historical account statements showing each purchase transaction for the shares sold
• For shares acquired through a dividend reinvestment plan (DRIP), the brokerage account history showing each reinvestment transaction with date and per-share price
• For inherited shares, documentation of the date of death and the fair market value per share on that date (the stepped-up basis)
• For gifted shares, documentation of the donor’s original basis and the date and value at the time of the gift
• Account transfer documentation if shares were moved between brokerages, confirming the original purchase information was carried over

Most major brokerages retain transaction history for 7 to 10 years through their online portals. If the purchase predates the online record, contact the brokerage’s records or tax department directly.

Step 3: Prepare the Response

If you have documentation establishing a cost basis that reduces the taxable gain, your response is a disagreement that includes:

• The Form 1099-B showing the gross proceeds
• Purchase confirmations or account statements establishing the cost basis for each lot sold
• A basis calculation table summarizing, for each security: proceeds, cost basis, and resulting taxable gain or loss
• The relevant pages from your Schedule D showing how you reported the sale

Your written response letter should state the correct gain figure (proceeds minus basis) and request that the IRS use this figure rather than treating the full proceeds as a gain.

Common Errors in Stock Sale Reporting

Noncovered Security Basis Not Documented

The most common error is not a reporting failure but a documentation failure: the taxpayer reported the correct gain on Schedule D but cannot produce the purchase records to verify the basis when challenged. Maintain brokerage records for all shares until at least three years after the year of sale.

Incorrect Cost Basis After a Stock Split or Corporate Action

If shares underwent a stock split, spin-off, merger, or other corporate action between purchase and sale, the basis must be adjusted accordingly. The original purchase price is divided across more shares after a split, which reduces the per-share basis. The brokerage should adjust the basis in its records, but verify this against the 1099-B for accuracy.

Wash Sale Disallowance Not Applied

If you sold a position at a loss and repurchased a substantially identical security within 30 days before or after the sale, the wash sale rules disallow the loss. The disallowed loss is added to the basis of the repurchased shares. If this adjustment was not made, the basis of the repurchased shares (and any future gain calculation) will be incorrect.

For guidance on assembling the document package for your response, see our article on CP2000 documents guide. For sample letter formats, see our CP2000 response letter templates.

Summary

Stock sale CP2000 notices most commonly arise because cost basis was not reported on the 1099-B, or because the basis on file differs from your actual purchase price. The response requires establishing the correct cost basis through purchase confirmations and account statements, preparing a basis calculation table, and submitting the documentation with a written explanation. A complete, well-organized basis dispute resolved through the CP2000 process typically succeeds without escalation.