Oddbean new post about login | logout What do you do when you can’t determine or prove the cost basis of a UTXO? Take the L and use $0 or will the IRS accept an educated estimate?
Hard to say what position the IRS would take. I'm not aware of any active cases to reference. I would say it doesn't have to tie back to an individual UTXO, but ideally some record of activity either from a wallet or exchange.