Imam al-Shafi'i (may Allah have mercy on him) said: Najsh is that a man is present at goods while they are being sold, and he offers a price for them although he does not want to buy them, so that the bargainers (bidders) follow him and give more for it than they would have given had they not heard his bid. Whoever raises the price in this way is a sinner if he knows of the prohibition of the Prophet (peace and blessings be upon him). The sale is valid; it is not invalidated by the disobedience of the man who raised the bid. In the time of the Messenger of Allah (peace and blessings be upon him), goods were sold to whoever offered more and the sale was valid, and it may be that the one who raised the bid did not intend to buy.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 10885