Positive commandment · Mitzvat Aseh #245 of 613
To render judgment [in questions] regarding business transactions
לדון בדין מקח וממכר
The verse · Vayikra 25:14
וְכִי־תִמְכְּרוּ מִמְכָּר לַעֲמִיתֶךָ אוֹ קָנֹה מִיַּד עֲמִיתֶךָ אַל־תּוֹנוּ אִישׁ אֶת־אָחִיו
“And if thou sell aught to thy neighbour, or buy aught of thy neighbour’s hand, you shall not defraud one another:”
The Rambam’s words
To render judgment [in questions] regarding business transactions, as [Leviticus 22:14] states: "If you sell merchandise to your colleague...."
לדון בדין מקח וממכר, שנאמר: "וכי תמכרו ממכר לעמיתך" (ויקרא כה,יד).
Why · Sefer HaChinuch #336
Adjudicating between the buyer and the seller
“The root of civil laws is well-known — as it is something to which human reason testifies and that it demands; for through law is civilization preserved among people.”
Who & when
“And [it] is practiced in every place and at all times by males, as it is upon them to administer justice.”
Where in the Mishneh Torah
Hilchot Sales הלכות מכירה