(68) بَابُ هَلْ يَبِيعُ حَاضِرٌ لِبَادٍ بِغَيْرِ أَجْرٍ وَهَلْ يُعِينُهُ أَوْ يَنْصَحُهُ
Narrated Tawus:
Ibn `Abbas said, "Allah's Messenger (ﷺ) said, 'Do not go to meet the caravans on the way (for buying their goods without letting them know the market price); a town dweller should not sell the goods of a desert dweller on behalf of the latter.' I asked Ibn `Abbas, 'What does he mean by not selling the goods of a desert dweller by a town dweller?' He said, 'He should not become his broker.' "
Reference: Sahih al-Bukhari 2158
In-book reference: Book 34, Hadith 109
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 3, Book 34, Hadith 367