That's the point of using blockchain because all the transactions are visible to anyone. NashX does not have any idea of Alice or Bob's identification, it only record the announced transction account and check the balance of each corresponding account to see if funds have arrived, all these can be done by program, and each time Alice and Bob can use different address

But there is no good way to prove that Bob has sent the USD to Alice, so there is still a risk of either Bod do not send or Alice do not acknowledge

Comparing with an escrow service, the Bob first send USD to escrow and after Alice saw the USD at escrow, she sent the BTC, and escrow will check the blockchain and confirm the transaction, then release the USD to Alice. So anyway fund should arrive at a public trusted location, either blockchain, escrow or exchange, when it comes to USD, you must have a USD account at escrow, then it is almost the same as exchange

And since NashX is an unregulated exchange, it will not be welcomed by the government