They used to buy food at the upper end of the market and then sell it on the spot. The Messenger of Allah ﷺ forbade them from selling it where they had bought it until they had transported it elsewhere.
كانوا يبتاعون الطعام في أعلى السوق، فيبيعونه في مكانهم فنهاهم رسول الله ﷺ أن يبيعوه في مكانه حتى ينقلوه.
Bukhari narrated it in "Sales" (2167) from Musaddad, who said, Yahya (bin Saeed Qattan) told us, from Ubaydullah, who said, Nafi told me, from Abdullah, and he mentioned it. Muslim also narrated it in "Sales" (1526) through another route from Ubaydullah with similar wording.