Reply Obj. 1: As stated above (I-II, Q. 96, A. 2) human law is given to the people among whom there are many lacking virtue, and it is not given to the virtuous alone. Hence human law was unable to forbid all that is contrary to virtue; and it suffices for it to prohibit whatever is destructive of human intercourse, while it treats other matters as though they were lawful, not by approving of them, but by not punishing them. Accordingly, if without employing deceit the seller disposes of his goods for more than their worth, or the buyer obtain them for less than their worth, the law looks upon this as licit, and provides no punishment for so doing, unless the excess be too great, because then even human law demands restitution to be made, for instance if a man be deceived in regard to more than half the amount of the just price of a thing [*Cod. IV, xliv, De Rescind. Vend. 2, 8].
On the other hand the Divine law leaves nothing unpunished that is contrary to virtue. Hence, according to the Divine law, it is reckoned unlawful if the equality of justice be not observed in buying and selling: and he who has received more than he ought must make compensation to him that has suffered loss, if the loss be considerable. I add this condition, because the just price of things is not fixed with mathematical precision, but depends on a kind of estimate, so that a slight addition or subtraction would not seem to destroy the equality of justice.
Reply Obj. 2: As Augustine says "this jester, either by looking into himself or by his experience of others, thought that all men are inclined to wish to buy for a song and sell at a premium. But since in reality this is wicked, it is in every man's power to acquire that justice whereby he may resist and overcome this inclination." And then he gives the example of a man who gave the just price for a book to a man who through ignorance asked a low price for it. Hence it is evident that this common desire is not from nature but from vice, wherefore it is common to many who walk along the broad road of sin.
Reply Obj. 3: In commutative justice we consider chiefly real equality. On the other hand, in friendship based on utility we consider equality of usefulness, so that the recompense should depend on the usefulness accruing, whereas in buying it should be equal to the thing bought. _______________________
SECOND ARTICLE [II-II, Q. 77, Art. 2]
Whether a Sale Is Rendered Unlawful Through a Fault in the Thing Sold?
Objection 1: It would seem that a sale is not rendered unjust and unlawful through a fault in the thing sold. For less account should be taken of the other parts of a thing than of what belongs to its substance. Yet the sale of a thing does not seem to be rendered unlawful through a fault in its substance: for instance, if a man sell instead of the real metal, silver or gold produced by some chemical process, which is adapted to all the human uses for which silver and gold are necessary, for instance in the making of vessels and the like. Much less therefore will it be an unlawful sale if the thing be defective in other ways.
Obj. 2: Further, any fault in the thing, affecting the quantity, would seem chiefly to be opposed to justice which consists in equality. Now quantity is known by being measured: and the measures of things that come into human use are not fixed, but in some places are greater, in others less, as the Philosopher states (Ethic. v, 7). Therefore just as it is impossible to avoid defects on the part of the thing sold, it seems that a sale is not rendered unlawful through the thing sold being defective.
Obj. 3: Further, the thing sold is rendered defective by lacking a fitting quality. But in order to know the quality of a thing, much knowledge is required that is lacking in most buyers. Therefore a sale is not rendered unlawful by a fault (in the thing sold).
On the contrary, Ambrose says (De Offic. iii, 11): "It is manifestly a rule of justice that a good man should not depart from the truth, nor inflict an unjust injury on anyone, nor have any connection with fraud."