It is forbidden to meet the caravans on the way (to buy the goods away from the market) Sahih al-Bukhari 2163
Arabic hadith text
حَدَّثَنِي عَيَّاشُ بْنُ الْوَلِيدِ، حَدَّثَنَا عَبْدُ الأَعْلَى، حَدَّثَنَا مَعْمَرٌ، عَنِ ابْنِ طَاوُسٍ، عَنْ أَبِيهِ، قَالَ سَأَلْتُ ابْنَ عَبَّاسٍ ـ رضى الله عنهما ـ مَا مَعْنَى قَوْلِهِ "لاَ يَبِيعَنَّ حَاضِرٌ لِبَادٍ ". فَقَالَ لاَ يَكُنْ لَهُ سِمْسَارًا
Translation
Narrated Tawus
I asked Ibn `Abbas, "What is the meaning of, 'No town dweller should sell (or buy) for a desert dweller'?" Ibn `Abbas said, "It means he should not become his broker."