I answer that, A threefold fault may be found pertaining to the thing which is sold. One, in respect of the thing's substance: and if the seller be aware of a fault in the thing he is selling, he is guilty of a fraudulent sale, so that the sale is rendered unlawful. Hence we find it written against certain people (Isa. 1:22), "Thy silver is turned into dross, thy wine is mingled with water": because that which is mixed is defective in its substance.
Another defect is in respect of quantity which is known by being measured: wherefore if anyone knowingly make use of a faulty measure in selling, he is guilty of fraud, and the sale is illicit. Hence it is written (Deut. 25:13, 14): "Thou shalt not have divers weights in thy bag, a greater and a less: neither shall there be in thy house a greater bushel and a less," and further on (Deut. 25:16): "For the Lord . . . abhorreth him that doth these things, and He hateth all injustice."
A third defect is on the part of the quality, for instance, if a man sell an unhealthy animal as being a healthy one: and if anyone do this knowingly he is guilty of a fraudulent sale, and the sale, in consequence, is illicit.
In all these cases not only is the man guilty of a fraudulent sale, but he is also bound to restitution. But if any of the foregoing defects be in the thing sold, and he knows nothing about this, the seller does not sin, because he does that which is unjust materially, nor is his deed unjust, as shown above (Q. 59, A. 2). Nevertheless he is bound to compensate the buyer, when the defect comes to his knowledge. Moreover what has been said of the seller applies equally to the buyer. For sometimes it happens that the seller thinks his goods to be specifically of lower value, as when a man sells gold instead of copper, and then if the buyer be aware of this, he buys it unjustly and is bound to restitution: and the same applies to a defect in quantity as to a defect in quality.
Reply Obj. 1: Gold and silver are costly not only on account of the usefulness of the vessels and other like things made from them, but also on account of the excellence and purity of their substance. Hence if the gold or silver produced by alchemists has not the true specific nature of gold and silver, the sale thereof is fraudulent and unjust, especially as real gold and silver can produce certain results by their natural action, which the counterfeit gold and silver of alchemists cannot produce. Thus the true metal has the property of making people joyful, and is helpful medicinally against certain maladies. Moreover real gold can be employed more frequently, and lasts longer in its condition of purity than counterfeit gold. If however real gold were to be produced by alchemy, it would not be unlawful to sell it for the genuine article, for nothing prevents art from employing certain natural causes for the production of natural and true effects, as Augustine says (De Trin. iii, 8) of things produced by the art of the demons.
Reply Obj. 2: The measures of salable commodities must needs be different in different places, on account of the difference of supply: because where there is greater abundance, the measures are wont to be larger. However in each place those who govern the state must determine the just measures of things salable, with due consideration for the conditions of place and time. Hence it is not lawful to disregard such measures as are established by public authority or custom.
Reply Obj. 3: As Augustine says (De Civ. Dei xi, 16) the price of things salable does not depend on their degree of nature, since at times a horse fetches a higher price than a slave; but it depends on their usefulness to man. Hence it is not necessary for the seller or buyer to be cognizant of the hidden qualities of the thing sold, but only of such as render the thing adapted to man's use, for instance, that the horse be strong, run well and so forth. Such qualities the seller and buyer can easily discover. _______________________
THIRD ARTICLE [II-II, Q. 77, Art. 3]
Whether the Seller Is Bound to State the Defects of the Thing Sold?
Objection 1: It would seem that the seller is not bound to state the defects of the thing sold. Since the seller does not bind the buyer to buy, he would seem to leave it to him to judge of the goods offered for sale. Now judgment about a thing and knowledge of that thing belong to the same person. Therefore it does not seem imputable to the seller if the buyer be deceived in his judgment, and be hurried into buying a thing without carefully inquiring into its condition.