Sahih Al Bukhari Hadith Number 2158

Narrated Tawus: Ibn `Abbas said, “Allah’s Messenger (peace be upon him) 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.’ “