Not a tax attorney but I have satisfied myself about the law on this point and this is what I do.
Cryptocurrency transactions are taxable when they are executed. If you have had the asset for > 1 year it is a long term gain/loss, otherwise a short term gain/loss.
If you trade one crypto for another you have to consider that you have sold one for cash, then bought the other for cash and calculate your taxable gain/loss on this basis. There is a lot of misinformation floating around to the effect that as long as you don't leave "crypto world" your gains are not taxable but this is (almost certainly) incorrect. It hinges on whether trading one crypto for another is a "like kind" transaction (which, by the way, you have to declare) and most tax attorneys seem to agree that it is not, any more than selling one stock and buying another which is fully taxable.
The IRS has not formally ruled on this but I'm pretty sure this is the way it will go. If you have claimed "like kind" status on your crypto transactions you will have to pay back taxes and interest and possibly penalties. If you haven't declared them at all I wouldn't want to be you.
It would make no sense for tax organizations to require an accounting of each switch from one altcoin to another, along with tax consideration for each switch. It would be like taxing a bean seller by each bean, and making that person fill out paper for each bean.
As long as fiat is the main currency, and until altcoins and bitcoin are widely used, it seems like common sense that the tax consideration involves "How much fiat went in" and "how much fiat came out". '
The last time I made money in Crypto was 2013 and I did not make enough to be taxed, by the standard of overall profit. In this current and upcoming bull run I probably will, and plan to pay taxes on the amount of dollar profit I actually made at the end of the day. Calculating transactions from different exchanges, each transaction in each currency, would take so much time, aside from making no sense, that it's only purpose would be to provoke more problems.
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Also, I'm not sure but I believe your comment about stocks is inaccurate. At least with regard to certain derivatives, there is something called 'basis cost', which is the total amount of money you spent on x countless number of trades, and you simply subtract the basis from what you have. Spend 10,000 on 500 trades, get 15,000 from those trades then basis cost is 10,000 subtracted from 15,000 equals 5000 taxable.
Where I think the problem will arise is in people who start businesses like localbitcoin. It is easily predictable that some petty bureaucrat will decide to spend 10s of millions of dollars hiring law enforcers to snoop through small businesses like that. America is already the heaviest "law enforcement" country in the world, meaning the most people in jail and the widest divide in accountability between those who go to jail and those who put others in jail. Along with law enforcement also will come a vast surge in thefts of coins by law enforcers. It is very safe to say that before too long most large thefts of coins will be by so called "law enforcers", and a pretty strong proof could be constructed to demonstrate why that is certain.