We must now consider those sins which relate to voluntary commutations. First, we shall consider cheating, which is committed in buying and selling: secondly, we shall consider usury, which occurs in loans. In connection with the other voluntary commutations no special kind of sin is to be found distinct from rapine and theft.
Under the first head there are four points of inquiry:
(1) Of unjust sales as regards the price; namely, whether it is lawful to sell a thing for more than its worth?
(2) Of unjust sales on the part of the thing sold;
(3) Whether the seller is bound to reveal a fault in the thing sold?
(4) Whether it is lawful in trading to sell a thing at a higher price than was paid for it?