Bukhari

Hadith Commentary:
Najsh specifically means to incite or provoke, especially in the context of hunting.
Here, it is used under a particular legal (shar‘i) meaning.
That meaning is that some merchants appoint deceitful agents whose sole task is to deceive buyers by any means possible and extract a higher price.
Such agents sometimes, in the presence of the customer, raise the price of the item and pretend to be buyers themselves,
even though they are not actual buyers.
The customer, being deceived, ends up purchasing the item at the inflated price.
In short, all forms of deception and fraud in sales are considered among the gravest major sins.
The Shari‘ah has strictly prohibited them.

Hadith Commentary:
(1)
The literal meaning of najsh is to drive game from its place so that it may be trapped in one’s net. In the Shariah, it means that a person bids up the price, even though he has no intention of buying. He only incites another to purchase the item and deceives him. Since the seller cooperates with him, both are partners in the sin.

(2)
In our commercial markets, traders appoint such agents whose job is to deceive every possible buyer and persuade them to offer a higher price. Sometimes these agents, in the presence of the buyer, raise the price of the desired item and pretend to be buyers themselves, even though they are not buyers. The customer, being deceived, ends up purchasing the item at a higher price. In short, all forms of deception in buying and selling are forbidden and are considered major sins. The Shariah has prohibited them.

Hadith Commentary:

That is, merely by lying to raise the price and deceiving customers, as auctioneers appoint agents who keep raising the price to deceive people.
This kind of deception is very evil.
How many poor people are robbed after falling into this deception.
Therefore, one should make every effort to avoid such trickery to the utmost extent.

Hadith Commentary:
➊
Najsh is to artificially raise the price of something without intending to purchase it, so that in this way, other people are encouraged to buy that item.
Since doing so is a trick to increase the price, the Messenger of Allah (sallallahu alayhi wa sallam) has prohibited it.
Nowadays, this very practice is common in our markets; thus, auctioneers raise the price merely by lying and deceive and defraud others.
In reality, these agents have no intention of buying the items.
Such deception is a very evil act—so many simple and poor people are robbed after falling into this trickery.
➋
The purpose of Imam Bukhari rahimahullah is that such trickery should never be employed during buying and selling, so that the poor public is not deceived.
And Allah knows best.

See Hadith: 4496, Benefit: 3.

Takhrij al-Hadith: [وأخرجه البخاري 2142، ومسلم 13/1516، من حديث مالك به]
Jurisprudential Explanation:
➊ In the language, the meaning of najsh is: “To artificially raise the price in an auction or sale, out of sympathy for the seller and to encourage purchase (without intending to buy oneself); this is called bay‘ muzayadah (auction sale), and it is religiously disliked (makruh).” (Al-Qamus al-Wahid, p. 1613, vol. J)
Imam Malik has also stated approximately the same meaning.
➋ If there is no intention to deceive in bidding, then it is permissible. See the previous hadith: 242
➌ It is narrated from Sayyiduna Anas ibn Malik (radi Allahu anhu) that an Ansari came and asked the Prophet (sallallahu alayhi wa sallam), so he (sallallahu alayhi wa sallam) said: “Do you not have anything in your house?” He replied: “Yes! A blanket, which we use to cover ourselves and also spread out, and a bowl from which we drink.” So he (sallallahu alayhi wa sallam) said: “Bring both of these here.” He brought them, and the Messenger of Allah (sallallahu alayhi wa sallam) took them in his hand and said: “Who will buy these items?” A man said: “I will buy both for one dirham.” He (sallallahu alayhi wa sallam) said two or three times: “Who will give more than one dirham?” Another man said: “I will buy both for two dirhams.” He (sallallahu alayhi wa sallam) took two dirhams from him and gave them to the Ansari... etc. [سنن ابي داود : 1641، وسنده حسن لذاته وحسنه الترمذي : 1218، ابوبكر الحنفي حسن الحديث ولم يصح قول البخاري فيه : ”لا يصح حديثه“ وأخطأ من ضعف هذا الحديث]
From this hasan li-dhatihi hadith, the permissibility of a legitimate auction is established.