“It should be conducted by a suitable person, such as a layman.
It should be for a necessary or pious cause, such as to provide for oneself and one’s family in need or to exercise works of mercy.
It should be done in a correct manner, namely without falsehood and perjury.
It should be held at a suitable time, such as not on a holiday.
It should take place in a location where such activity is permitted and appropriate, such as in a market and not in a church.
It should use pricing by a just estimation of the goods, and by trade, according to how it is usually sold in that city or region in which business is conducted.”(1)
(1) From Odd Langholm, Economics in the Medieval Schools: Wealth, Exchange, Value, Money and Usury According to the Paris Theological Tradition 1200-1350 (Leiden: E.J. Brill), 1992, p. 137.