There are a few pieces to this answer: 

First, in a democracy, there is no higher authority than the citizen and, since a CA would be composed of ordinary citizens, they would be accountable to each other. 

Second, membership is temporary so they would hold themselves accountable. With nothing to gain or lose politically but the security and prosperity of their own community, which they will soon be returning to, they have every incentive to act as conscientious citizens. 

Third, assembly members would still be subject to the rule of law. Members who commit crimes, like bribery for example, would be held accountable by the justice system.