Earlier quoted context omitted.
Treat it as if you sold the crypto for USD (and pay capital gains). So if you bought a crypto for $10, the price appreciated to $110, and then you bough something worth $110, then you have to pay taxes on $100 of income. It’s the same place you repot capital gains for equity (I don’t remember the form number)
What if I bought 1BTC at $10, then I bought 1BTC at $200, then I bought something with 1BTC when BTC dropped to $100/1BTC? Which BTC was used for the purchase- the one I bought at $10 or the one at $200? It sort of kills the use of cryptocurrency as "currency" in the US. Imagine if in the end of the year we had to report every dollar transaction (e.g. buying a coffee and a bagel), trace how you earned that dollar and…
Edit: easiest thing to do as a workaround, imo, would be to build tax reporting into wallet software.