The townsman'should not sell for a man from the desert (with a view to taking advantage of his ignorance of the market conditions of the city). And Zuhair reported from the Prophet ﷺ that he forbade the townsman to sell on behalf of the man from the desert.
النَّبِيَّ ﷺ قَالَ لاَ يَبِعْ حَاضِرٌ لِبَادٍ وَقَالَ زُهَيْرٌ عَنِ النَّبِيِّ ﷺ أَنَّهُ نَهَى أَنْ يَبِيعَ حَاضِرٌ لِبَادٍ
The Messenger of Allah ﷺ forbade a townsman to sell for a man from the desert. I asked: What do you mean by the selling of a townsman for a man from the desert ? He replied: He should not be a broker for him.
نَهَى رَسُولُ اللَّهِ ﷺ أَنْ يَبِيعَ حَاضِرٌ لِبَادٍ فَقُلْتُ مَا يَبِيعُ حَاضِرٌ لِبَادٍ قَالَ لاَ يَكُونُ لَهُ سِمْسَارًا
[Machine] I heard Abu Huraira saying that the Messenger of Allah ﷺ forbade buying a captive for a ransom. And Abu Nuaim said, "He does not sell a captive for a ransom."
سَمِعْتُ أَبَا هُرَيْرَةَ يَقُولُ نَهَى رَسُولُ اللهِ ﷺ أَنْ يَشْتَرِيَ حَاضِرٌ لِبَادٍ وَقَالَ أَبُو نُعَيْمٍ لَا يَبِيعُ حَاضِرٌ لِبَادٍ
[Machine] The Messenger of Allah ﷺ prohibited selling a product that is not in the possession of the seller and said, "Do not engage in present transactions."
نَهَى رَسُولُ اللَّهِ ﷺ أَنْ يَبِيعَ حَاضِرٌ لِبَادٍ وَقَالَ «لَا تَلَقُّوا الْبُيُوعَ»