It was narrate from Ibn Tawus, from his father, that Ibn 'Abbas said; "The Messenger of Allah forbade meeting the riders, and for a town-dweller." I said to Ibn 'Abbas:"What does a town-dweller (selling) for a desert-dweller mean?" he said: "He should not act as a broker for him,"
أَخْبَرَنَا مُحَمَّدُ بْنُ رَافِعٍ، قَالَ أَنْبَأَنَا عَبْدُ الرَّزَّاقِ، قَالَ أَنْبَأَنَا مَعْمَرٌ، عَنِ ابْنِ طَاوُسٍ، عَنْ أَبِيهِ، عَنِ ابْنِ عَبَّاسٍ، قَالَ نَهَى رَسُولُ اللَّهِ صلى الله عليه وسلم أَنْ يُتَلَقَّى الرُّكْبَانُ وَأَنْ يَبِيعَ حَاضِرٌ لِبَادٍ . قُلْتُ لاِبْنِ عَبَّاسٍ مَا قَوْلُهُ حَاضِرٌ لِبَادٍ قَالَ لاَ يَكُونُ لَهُ سِمْسَارًا .
Nesai
The Book of Financial Transactions • Hadith 4500