The IRS can't tell how long you've held a bitcoin, what price you bought it at, when you bought it or how many bitcoins you hold in total. The fact that you can

The IRS can't tell how long you've held a bitcoin, what price you bought it at, when you bought it or how many bitcoins you hold in total. The fact that you can arbitrarily split wallets and tumble coins means they basically have to trust what you say, or accuse you of fraud. This is similar to how you could melt down gold and lie about when you bought it and the price you bought it at.