But they really have no way to know whether you are lying. As a result, people lie about the value of traded goods, or art, or land properties, or unlisted financial instruments, to reduce the amount of income tax they purportedly owe. This is a major tax-evasion (not avoiding) and/or money-laundering loophole employed by the rich, especially when employing art and real estate, which may be justifiably non-comparable to similar goods due to uniqueness.

A law-obeying person would liquidate enough of the subjective-value goods to pay income tax at the maximum withholding rate at the time of receipt, and send that amount to the IRS at the end of the quarter, then claiming a refund from that amount with their return at the end of the year.

A practical, law-breaking person would just keep their mouth shut about it, and allow the IRS to claim it was income that had value, and only pay taxes on it (or dispute the amount demanded) if the IRS actually demanded an amount.

The enforcement on Bitcoin-holders is not to raise revenue in any meaningful sense. It is to discourage use of cryptocurrencies as a means of tax evasion--probably because middle-class people could make use of it. With respect to the means employed by the rich to evade and avoid taxes, an equivalent effort would likely return 1000 times greater rewards.