Such exchanges can be totally anonymous, including who runs them. They don't deal with people or fiat currency. Just usernames and blockchains. People trust them based on their history, like an ebay seller, and admins realize there's no need to rip off users because it's not worth it to them in the long run (let alone what might happen to them if they are personally outed for doing so.) People can go in and out of them for very short periods of time to lower the risk it should disappear during that time, ie to lower their "exchange risk." They could also have a set lifespan... "we will operate from 2015-2018" this might lower risks as well.. imagine if Silk Road had a fixed shelf life that expired before it was caught.