Bittrex don't exchange fiat currencies so they have nothing to report except there own business tax returns.
I don't think that's the case. I mean, they don't report to the IRS. But virtually every transaction (including crypto-crypto) is taxable from a capital gains perspective. Just because you don't wire dollars to your bank account doesn't mean you don't technically owe taxes. This is why I prefer not to verify ID at exchanges. Just in case...