The Catechism of the Summa - SECOND SECTION - A DETAILED SURVEY OF MAN'S RETURN TO GOD - OF THE SINS THAT DECEIVE OUR NEIGHBOUR, AND OF THOSE THAT TAKE ADVANTAGE OF HIM, VIZ., OF FRAUD AND OF USURY (A)

What is the last kind of sins which are committed against commutative justice?

They are those sins whereby our neighbour is induced to agree to things that are prejudicial to him (LXXVII., Prologue).
 
What are these sins called?

They are called fraud and usury (LXXVII., LXXVIII.).

(A)  
What is fraud?

It is an act of injustice, which is committed in buying and selling whereby our neighbour is deceived and is led to will what is to his loss (LXXVII.).
 
In how many ways can one commit the sin of fraud?

This sin may be committed: by reason of the price which is more than the thing is worth; by reason of the thing sold in so far as it is not what it appears to be, the seller knowing this and the buyer ignoring it; by reason of the seller who conceals a defect in the object sold; and by reason of the end in view (LXXVII. 1-4.).
 
Is it never permitted knowingly to sell a thing for more than it is worth or to buy a thing for less than it is worth?

No, for the price of a thing bought or sold must always correspond to the worth of the thing itself; to ask more or to give less knowingly is of itself essentially unjust, and obliges to restitution (LXXVII. 1).
 
Is it against justice to sell a thing for what it is not, or to buy a thing which is other than what the seller believes it to be?

Yes, to sell or to buy a thing which is other than it appears to be, whether there be question of its substance, or its quantity or quality, is contrary to justice, and is a sin if one do this knowingly; moreover, one is bound to make restitution. Further, this obligation of making restitution exists even when there has been no sin, as soon as ever one discovers the disproportion between the price and the thing sold or bought (LXXVII. 2).
 
Is the seller always bound to bring to the notice of the buyer the defects of his wares in so far as he knows them?

Yes, he is bound to do this whenever the defects are hidden, and might be a cause of danger or loss to the buyer (LXXVII. 3).
 
Is it allowable to take up buying and selling as a form of trade for the sake of gaining money?

Trading for the sake of trading is a shameful thing and contrary to justice; because of itself it promotes the love of lucre, which knows no limits, but seeks to acquire without end (LXXVII. 4).
 
What then is necessary that trading may become licit and honest?

Lucre should not be sought after for its own sake but for some good end. In this way one may seek a moderate gain by trading in order to maintain one's household, or to give help to the needy, or one may do this for the public benefit to the end that one's fellow-men may not lack the things necessary for daily life, and one may seek such gain not as an end in itself but as the recompense for one's work (LXXVII. 4).

Next - The Catechism of the Summa - SECOND SECTION - A DETAILED SURVEY OF MAN'S RETURN TO GOD - OF THE SINS THAT DECEIVE OUR NEIGHBOUR, AND OF THOSE THAT TAKE ADVANTAGE OF HIM, VIZ., OF FRAUD AND OF USURY (B)