It is narrated from Sayyiduna Jabir (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "No townsman should sell goods on behalf of a villager; rather, let the people sell for themselves so that Allah, the Exalted, may provide sustenance to them from one another."
Hadith Referenceالسنن المأثورة / باب في البيوع / 255
Hadith Takhrijصحیح مسلم، البيوع، باب تحريم بيع الحاضر للبادى رقم: 1522