Riyad us Saliheen: Hadith 1776

The Messenger of Allah (ﷺ) said, "The caravans carrying merchandise should not be met on the way to purchase from them; a man in the city should not sell for a man of the desert."

Tawus asked him (Ibn 'Abbas): "What do these words really imply?" He said: "He should not work as an agent on his behalf."

[Al-Bukhari and Muslim].