It is narrated from Sayyiduna Talha bin Ubaidullah (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) forbade that a townsman should trade on behalf of a villager. This is a lengthy hadith.
Hadith Referenceالفتح الربانی / كتاب البيوع والكسب والمعاش وما يتعلق بالتجارة / 5898