At the very least you would have to register as a money services business, which Poloniex and I believe Bittrex both have, and then as a MSB you would have to perform KYC/AML and record keeping to comply with the Bank Secrecy Act to detect and prevent money laundering. IANAL.
Its very difficult and very expensive in the US. And even if you pay and jump through all the hoops, nothing is certain. This is how banks keep the competition out. I would suggest looking into more friendly countries abroad, or consider decentralized solutions.