d not be
more of a burden to one party than to another, and consequently all
contracts between them should observe equality of thing and thing.
Again, the quality of a thing that comes into human use is measured
by the price given for it, for which purpose money was invented, as
stated in _Ethic._ v, 5. Therefore if either the price exceed the
quantity of the thing's worth, or, conversely, the thing exceed the
price, there is no longer the equality of justice: and consequently,
to sell a thing for more than its worth, or to buy it for less than
its worth, is in itself unjust and unlawful.

Secondly we may speak of buying and selling, considered as
accidentally tending to the advantage of one party, and to the
disadvantage of the other: for instance, when a man has great need of
a certain thing, while another man will suffer if he be without it.
In such a case the just price will depend not only on the thing sold,
but on the loss which the sale brings on the seller. And thus it will
be lawful to sell a thing for more than it is worth in itself, though
the price paid be not more than it is worth to the owner. Yet if the
one man derive a great advantage by becoming possessed of the other
man's property, and the seller be not at a loss through being without
that thing, the latter ought not to raise the price, because the
advantage accruing to the buyer, is not due to the seller, but to a
circumstance affecting the buyer. Now no man should sell what is not
his, though he may charge for the loss he suffers.

On the other hand if a man find that he derives great advantage from
something he has bought, he may, of his own accord, pay the seller
something over and above: and this pertains to his honesty.

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 restitutio