Sahih Bukhari Volume 3, Book 36, Hadith Number 474.

” Narated By Tawus : Ibn ‘Abbas said, “The Prophet forbade the meeting of caravans (on the way) and ordained that no townsman is permitted to sell things on behalf of a bedouin.” I asked Ibn ‘Abbas, “What is the meaning of his saying, ‘No townsman is permitted to sell things on behalf of a bedouin.’ ” He replied, “He should not work as a broker for him.