Sahih Bukhari Volume 3, Book 34, Hadith Number 367.

Narated 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.'”