Roth IRAs, just like traditional IRAs, fall in the category of "tax-deferred" accounts. That means that as long as money stays in the account, it's not subject to the normal tax laws.
So any transactions you make inside that account are tax-free. But they are also not subject to other tax laws, such as deducting losses. Just as you can't be taxed on profitable sales if they stay in the account, you can't deduct losing sales.
In short, none of the regular tax laws apply until you start taking money out of the account. Then those withdrawals are taxed as ordinary income.