Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) says that the Messenger of Allah (peace and blessings be upon him) forbade "that the trade caravans should be met outside the town, and that a town-dweller should sell for a desert-dweller." The narrator says: I said to Ibn Abbas (may Allah be pleased with them both): What is the meaning of his words "No town-dweller should sell for a desert-dweller"? He said: That he should not become his broker.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 10914