Wow, never heard that. Why would shepherds, whose time and energy is spent tending flocks sell bread? They did not have the preservatives and processes we have to day to keep bread fresh and if someone was going to sell bread, why would it be vagabond shepherds? It was most likely farmers who did not move around much and had means to store grain until time to grind and make the bread or merchants who had grain storage facilities and could take advantage of the process. From what I understand about shepherds they were moving around a lot so it just wouldn't make sense.