From Sahih Muslim (Hadith 3825):
Ibn 'Abbas (Allah be pleased with them) reported Allah's Messenger (ﷺ) as saying:The riders (carrying merchandise) should not be met in the way, and townsman should not sell for a man of the desert. The narrator reported. I said to Ibn 'Abbas: What do these words really imply-" The townsman for the man of the desert"? He said: That he should work as a broker on his behalf
Reference: Muslim 3825