Hadith: The Messenger of Allah (may Allah's peace and blessings be upon him) forbade the town-dweller to sell (merchandise) on behalf of a Bedouin

عن أبي هريرة رضي الله عنه : «نهى رسول الله صلى الله عليه وسلم أن يبيع حاضرٌ لباد، ولا تناجشوا. ولا يبَعِ ِالرجل على بيع أخيه، ولا يخطب على خطبته ، ولا تسأل المرأة طلاق أختها لتِكْفَأَ ما في صَحفَتِهَا».
[صحيح] - [متفق عليه]

Abu Hurayrah (may Allah be pleased with him) reported: The Messenger of Allah (may Allah's peace and blessings be upon him) forbade the town-dweller to sell (merchandise) on behalf of a Bedouin. And not to practice Najsh (offering a high price for a thing which you do not really want to buy in order to raise its price value and deceive other buyers). No Muslim should offer more for a thing already bought by his Muslim brother, nor should he propose to a women already engaged to another Muslim. A Muslim woman should not ask that her sister (i.e. another Muslim woman) be divorced in order to invert her plate (i.e. an idiom meaning to deprive her of what she has in order to take her place).
Sahih/Authentic. - [Al-Bukhari and Muslim]

Explanation

The beneficial and wholesome teachings of Islam seek to eliminate all means that may cause enmity and hatred among Muslims. This is obvious in the following prohibitions mentioned in this Hadīth: The Prophet (may Allah's peace and blessings be upon him) prohibited raising the price of an item without the true intention of buying it. Some people may do this seeking to benefit of the seller by increasing the price (in front of other buyers), or to harm the actual buyers by making the item become too expensive for him to buy. He prohibited this action because it comprises of falsehoods and lying to the customers and cheating them, and raising the price of merchandise by way of trickery and deception. He also prohibited the city or town dweller to sell merchandise on behalf of the Bedouin because the city dweller knows well the prices of the merchandise while the Bedouin does not and thus the latter might mistakenly sell it at a lower price and the buyers would unfairly benefit from that price difference. The Prophet (may Allah's peace and blessings be upon him) said in this regard: "Leave people (to engage in trade) and Allah will grant them provision through one another." If the Bedouin sells the merchandise himself this will benefit the buyers more. Thus the prohibition that a city dweller sells on behalf of a Bedouin is intended to eliminate harmful consequences from city residents. He also prohibited that a man proposes to marry a woman to whom another man has already proposed until he learns that the other suitor was clearly turned down. This is prohibited because of the obvious enmity and hatred it would cause between the two suitors, and what it involves of depriving a fellow Muslim of something good. He also prohibited that a woman asks her husband to divorce his other wife, or turns him against her or cause quarrels between them so that he eventually divorces her. This is impermissible because of the evil it involves. It creates enmity and deprives the divorced wife of the benefits and good that she enjoyed in her marriage as her husband was obliged to provide for her and clothe her, as well as all the other marital rights. This state of deprivation was expressed in the Hadīth with the idiom and metaphor: "inverting what's on her plate" to express depriving her of the good she enjoys while married.