First, many bitcoin users "own" their bitcoins using a managed web wallet like Coinbase. In that case, you cannot trace the individuals coins on the blockchain.

First, many bitcoin users "own" their bitcoins using a managed web wallet like Coinbase. In that case, you cannot trace the individuals coins on the blockchain. It is up to the tax payer to report their cost basis. My broker handles stock in a very similar way. My shares of stock are held in street name by my broker, and my name is not directly assciated with individual shares. Instead, the broker keeps the shares and has a separate ledger keepting track of how many I own. So far, the IRS hasn't used this situation as an excuse to pwn me. I don't think they'd even see anything particularly remarkable about it. After all, it's standard operating procedure for essentially every brokerage.