Objection 1. It would seem that it is not lawful, in trading, to sell a thing for a higher price than we paid for it. For Chrysostom [Hom. xxxviii in the Opus Imperfectum, falsely ascribed to St. John Chrysostom] says on Matthew 21:12: "He that buys a thing in order that he may sell it, entire and unchanged, at a profit, is the trader who is cast out of God's temple." Cassiodorus speaks in the same sense in his commentary on Psalm 70:15, "Because I have not known learning, or trading" according to another version [the Septuagint]: "What is trade," says he, "but buying at a cheap price with the purpose of retailing at a higher price?" and he adds: "Such were the tradesmen whom Our Lord cast out of the temple." Now no man is cast out of the temple except for a sin. Therefore such like trading is sinful.
Objection 2. Further, it is contrary to justice to sell goods at a higher price than their worth, or to buy them for less than their value, as shown above (Article 1). Now if you sell a thing for a higher price than you paid for it, you must either have bought it for less than its value, or sell it for more than its value. Therefore this cannot be done without sin.
Objection 3. Further, Jerome says (Ep. ad Nepot. lii): "Shun, as you would the plague, a cleric who from being poor has become wealthy, or who, from being a nobody has become a celebrity." Now trading would net seem to be forbidden to clerics except on account of its sinfulness. Therefore it is a sin in trading, to buy at a low price and to sell at a higher price.
On the contrary, Augustine commenting on Psalm 70:15, "Because I have not known learning," [Cf. Objection 1 says: "The greedy tradesman blasphemes over his losses; he lies and perjures himself over the price of his wares. But these are vices of the man, not of the craft, which can be exercised without these vices." Therefore trading is not in itself unlawful.
I answer that, A tradesman is one whose business consists in the exchange of things. According to the Philosopher (Polit. i, 3), exchange of things is twofold; one, natural as it were, and necessary, whereby one commodity is exchanged for another, or money taken in exchange for a commodity, in order to satisfy the needs of life. Such like trading, properly speaking, does not belong to tradesmen, but rather to housekeepers or civil servants who have to provide the household or the state with the necessaries of life. The other kind of exchange is either that of money for money, or of any commodity for money, not on account of the necessities of life, but for profit, and this kind of exchange, properly speaking, regards tradesmen, according to the Philosopher (Polit. i, 3). The former kind of exchange is commendable because it supplies a natural need: but the latter is justly deserving of blame, because, considered in itself, it satisfies the greed for gain, which knows no limit and tends to infinity. Hence trading, considered in itself, has a certain debasement attaching thereto, in so far as, by its very nature, it does not imply a virtuous or necessary end. Nevertheless gain which is the end of trading, though not implying, by its nature, anything virtuous or necessary, does not, in itself, connote anything sinful or contrary to virtue: wherefore nothing prevents gain from being directed to some necessary or even virtuous end, and thus trading becomes lawful. Thus, for instance, a man may intend the moderate gain which he seeks to acquire by trading for the upkeep of his household, or for the assistance of the needy: or again, a man may take to trade for some public advantage, for instance, lest his country lack the necessaries of life, and seek gain, not as an end, but as payment for his labor.
Reply to Objection 1. The saying of Chrysostom refers to the trading which seeks gain as a last end. This is especially the case where a man sells something at a higher price without its undergoing any change. For if he sells at a higher price something that has changed for the better, he would seem to receive the reward of his labor. Nevertheless the gain itself may be lawfully intended, not as a last end, but for the sake of some other end which is necessary or virtuous, as stated above.
Reply to Objection 2. Not everyone that sells at a higher price than he bought is a tradesman, but only he who buys that he may sell at a profit. If, on the contrary, he buys not for sale but for possession, and afterwards, for some reason wishes to sell, it is not a trade transaction even if he sell at a profit. For he may lawfully do this, either because he has bettered the thing, or because the value of the thing has changed with the change of place or time, or on account of the danger he incurs in transferring the thing from one place to another, or again in having it carried by another. On this sense neither buying nor selling is unjust.
Reply to Objection 3. Clerics should abstain not only from things that are evil in themselves, but even from those that have an appearance of evil. This happens in trading, both because it is directed to worldly gain, which clerics should despise, and because trading is open to so many vices, since "a merchant is hardly free from sins of the lips" ['A merchant is hardly free from negligence, and a huckster shall not be justified from the sins of the lips'] (Sirach 26:28). There is also another reason, because trading engages the mind too much with worldly cares, and consequently withdraws it from spiritual cares; wherefore the Apostle says (2 Timothy 2:4): "No man being a soldier to God entangleth himself with secular businesses." Nevertheless it is lawful for clerics to engage in the first mentioned kind of exchange, which is directed to supply the necessaries of life, either by buying or by selling.
Source: Summa Theologica (New Advent)