Since you are not from the US, I can tell you a bit about how this actually works. The US has a dual reporting system. Meaning the both the broker and the taxpayer report independently to the IRS.
If the dually reported numbers match, then the IRS believes the broker's numbers and does not look at the details.
The dual reporting mechanism is actually fairly ingenious. This is because usually both parties have opposite incentives to cheat if they choose to cheat. For example, a business has the incentive to cheat by claiming they paid more wages. But the employees have the incentive to cheat by claiming that they were paid less wages. This opposing incentive system actually results in almost all cases reporting the correct numbers. A lot of the tax fraud occurs when the dual parties are the same person (such as a sole proprietorship).
1) Brokers are required to send you and the IRS a form that lists the
total purchases,
total sales, and
total profit. See lines 1 and 8 here:
2) The taxpayer is required to send the IRS a form that lists the
total purchases,
total sales, and
total profit. See line 1a here:
3) The taxpayer
may optionally fill out an extended form listing individual sales, but even that optional form has at the very bottom the
total purchases,
total sales, and
total profit. See line 2 here:
So, ultimately, the broker and the tax filer have to do all that work and go through all the papers. The IRS just receives a copy in case your numbers do not match the broker's numbers. And in that case, you'd be lying on your tax forms and will be audited. Sure, the audit will cost the IRS time. But it can also cost you an awful lot.