I haven't used bittrex for a while now since they started asking for KYC, but this is completely crazy. If I were you, I'd ask them to unlock my bitcoins and cancel all my trades, with bitcoins sent back to your wallet or back to where they came from if that's acceptable. I would never get into such a ridiculous back and forth discussion with them.
Basically what they are doing is asking you to prove how you were able to obtain the coins that you are trying to sell, which is unacceptable to a holder like me, who has no proof of purchase and no incentive to give them my bank statements or business records, especially since my coins were never bought through a bank but earned or bought with cash. Just find yourself another exchange and request for your bittrex account to be closed and funds returned. You have made a mistake of depositing your coins with bittrex and you should correct it ASAP. If they are afraid of money laundering, they can either deny you services (but can't hold your coins because that's theft), or report you to the authorities for verification.
If they keep playing cat and mouse with you and holding your coins, get a lawyer.
