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A form 1099-B would be filed with the IRS for any brokerage account by the exchange (I.e. e-trade, Vanguard, etc). This would report any account proceeds (and usually cost basis as well) directly to the IRS.

If the account-holder fails to include the proceeds in their tax return, the IRS knows (from Brokerage) that the filer forgot/omitted the proceeds on the tax return and it will be flagged for a letter/review/audit/etc.

Instructions for Form 1099B - IRS.gov
https://www.irs.gov/pub/irs-pdf/i1099b.pdf

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