Genesis 42:6

‘And Joseph was the governor over the land, he it was who sold to all the people of the land, and Joseph's brothers came and bowed themselves to him with their faces to the earth.'

It is probable that Joseph had arranged things in such a way that all Canaanites coming to buy food had to approach him. He would not of course be actually selling the food but would be on a seat of honour and approached by those who came, who would abase themselves to him before passing on to those who actually handled the transactions.