Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) says that in the time of the Messenger of Allah (peace and blessings be upon him) they used to buy goods from the trade caravans, and he (peace and blessings be upon him) forbade them to sell them in the place where they had bought them, until they were moved to the market.
(b) In the narration of Juwayriyah, Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) says: We used to go out of the town to meet the trade caravans and buy grain from them, and the Messenger of Allah (peace and blessings be upon him) said: "Do not sell it before taking it to the market."
Note: Buying goods from the trade caravans is valid; it was forbidden only on account of possession, so that they would move it, since the grain may be of a higher price in another place than in that place.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 10921