Sahih Bukhari | Volume 3 | Hadith Number 367
Narrated by Tawus
Ibn 'Abbas said, "Allah's Apostle 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.'"