According to Catholic teaching, the seller must be honest about the sale of his goods as a matter of justice. He must not deceive the buyer or claim the goods are more than they really are. The seller must also inform the buyer of known defects that would have an impact on the value of the product in question.
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The question arises if the seller is obliged to point out obvious defects that are clearly visible such as a dent in the fender of a car.
The answer given by medieval moral theologians is commonsensical. Economist Odd Longholm summarizes their conclusions:
“A seller is obliged to reveal hidden defects in goods if they are likely to 
cause danger or loss. He is not obliged to point out manifest defects because this might make it difficult to obtain a price that is just considering the defect. The seller may look to his own indemnity by withholding information about the defect.” (Odd Langholm, The Merchant in the Confessional: Trade and Price in the Pre-Reformation Penitential Handbooks, (Leiden; Boston: Brill) 2003, p. 55.)
In other words, a seller who insistently points out an obvious defect might deprive himself of the just price that is due to him. The buyer seeing the defect knows what he is buying and thus need not be advised.