Cicero starts from the premise that we want to be honest but that it may not always be obvious what an honest person should do in certain business situations. Now Cicero was very much a man of the world. He knew first hand that not everyone is honest. That, however, is morally an easy matter to address. Dishonesty is wrong.
Dishonest people deserve our contempt and censure, perhaps even punishment.
Here's the first test case. A Famine at Rhodes Suppose that there is a food shortage and famine at Rhodes, and the price of corn is extremely high. An honest man has brought the Rhodians a large stock of corn from Alexandria. I am assuming he is an enlightened, honest person. I am asking you to consider the deliberations and self searchings of the sort of man who would not keep the Rhodians in ignorance if he thought this would be dishonest but who is not certain that dishonesty would be involved.