Hadith: Do not go out to meet the riders (in a trade caravan), do not urge buyers to cancel a sale transaction to make a new one with you, do not bid against each other (to fool another bidder), a townsman must not buy on behalf of a Bedouin, and do not tie up the udders of camels and sheep

عن أبي هريرة رضي الله عنه عن النبي صلى الله عليه وسلم : «لا تَلَقَّوُا الرُّكبان، ولا يبع بعضكم على بيع بعض، ولا تَنَاجَشُوا ولا يبع حَاضِرٌ لِبَادٍ، ولا تُصَرُّوا الإبلَ والغنم، ومن ابتاعها فهو بخير النَّظَرَين بعد أن يحلبها: إن رَضِيَهَا أمسكها، وإن سَخِطَها رَدَّهَا وَصَاعا ًمن تمر».
[صحيح] - [متفق عليه، والرواية الثانية رواها مسلم]

Abu Hurayrah (may Allah be pleased with him) reported that the Prophet (may Allah's peace and blessings be upon him) said: "Do not go out to meet the riders (in a trade caravan), do not urge buyers to cancel a sale transaction to make a new one with you, do not bid against each other (to fool another bidder), a townsman must not buy on behalf of a Bedouin, and do not tie up the udders of camels and sheep (so that they appear to have a lot of milk). However, if someone buys them while in this condition and has already milked them, then he has two options: If he is pleased with them, he may keep them, and if he is displeased with them, he may return them along with a Sā‘ of dates.”
[Authentic hadith] - [Narrated by Muslim - Narrated by Bukhari & Muslim]

Explanation

The Prophet (may Allah's peace and blessings be upon him) forbids five types of sale transactions, due to the harm involved in them that would befall the buyer or the seller or other parties. 1. He forbade people from going out to meet the trade caravans coming to sell their merchandise including foodstuff and animals, aiming thereby to meet them before they arrive at the local market and to buy from them taking advantage of their ignorance of the prices. They may, thus, be easily cheated and deprived of their full profit. 2. He also prohibited urging the buyer or seller to cancel the transaction that has already been made so as to take the deal for oneself. This occurs when someone tells the buyer, while still in the contracting session, when the option to cancel the transaction is still in effect, that he is ready to offer him something of better quality, or a better price. Or if someone is selling an item, he would tell him that he will buy it for a higher price. He does this only to cancel the current transaction and take it for himself. It is also prohibited to do this after there is no longer an opportunity to cancel the transaction by virtue of “Khiyār al-Majlis” (an option whereby the parties have right to withdraw from a contract so long as the parties do not leave the place of contract) and “Khiyār Ash-Shart” (an option to cancel a previous agreed upon sale within a specific number of days), due to the disputation, animosity and hatred that this provocation may lead to, and the damage it causes to people’s livelihood. 3. The Messenger of Allah (may Allah’s peace and blessings be upon him) also prohibited Najsh, which is bidding on a commodity without the intention of buying it, but rather to aid the seller by increasing the bidding price, or to cause damage to the buyer by making the merchandise more expensive for him to buy. The Prophet (may Allah's peace and blessings be upon him) forbade this action owing to the lying and deceiving of the buyers, and because of raising the commodity’s price by way of deception and fraud. 4. The Messenger of Allah (may Allah’s peace and blessings be upon him) also prohibited a resident of a city to sell commodities on behalf of a Bedouin. This is because the townsman knows the prices better than the Bedouin, so the people of the city would not be able to benefit from the lower prices the Bedouin would offer them. The Messenger of Allah (may Allah’s peace and blessings be upon him) said: “Leave people alone, Allah will give them provision through one another.” 5. The prohibition of selling cattle after tying up their udders so that the buyer thinks that the animal always has that much milk, and so he buys it for a higher price. The Shariah has given the buyer a three-day period in which he can retrieve his right. He may keep it if he so wishes, or return it to the seller after he has known that its udder was tied up. If he milked it while it was with him, and wishes to return it, then he can do so along with offering one Sā‘ of dates, in place of the milk he got from it.