When you sell ESPP shares, the IRS taxes your gain: sale price – cost basis. That cost basis should be the full market price on the purchase date, because you’ve already paid ordinary income tax on the discount you got. Most brokerages, however, report only the discounted price. If your company’s share price has risen, that gap can be huge—so the IRS thinks you made a bigger profit than you really did and you end up paying tax twice on the same dollars. Our free review spots this mismatch and shows how much you can get back.