Musar zu Mischlej 10:4

Consider the prohibition of Onaah (fraud/deceit). How easy it is for a person to be enticed (to find permits - SP) and to stumble in it (actual sin - SP). For it appears to him proper to strive to make his merchandise appear appealing to the eyes of people in order to make a profit from the toil of his hands; to try to speak to the prospective customer's heart in order to convince him (salesman talk). He will justify himself on all this citing: "some are industrious and profit" (Pesachim 50b) or "the hand of the diligent prospers" (Mishlei 10:4).

Likewise regarding the character traits, it is necessary for him to read the teachings of mussar, whether of the early or later sages. For very often, even after one has resolved to become among the meticulously Clean, he may possibly sin in details he never attained knowledge in.