There are at least two reasons why it is very difficult to define ethical behavior in the context of business dealings.
First, there are so many kinds of dealings that businesspeople will have that it is very difficult to make rules of ethical behavior that will actually cover all of them. Any rule that will cover many situations must necessarily be quite general and will be hard to apply to a given situation.
Second, business is an area in which even ethical behavior harms people. An ethical behavior on my part that increases my share of a market will harm my competitors by taking business from them. Thus, the whole area of endeavor is aimed in some ways at hurting other people. In such a case, defining ethics becomes very difficult.