As we have learned, the Mishnah at the beginning of this perek (=chapter, see daf, or page 83) teaches that only the choicest produce was to be used for Temple offerings. The second-tier city for produce mentioned in the Mishnah was Aforayim. The Gemara on today’s daf attempts to show how common grain was in this city from the following midrashicstory.
When Moshe approached the Egyptian Pharaoh armed with the magical feats that had been prepared for him by God (see Shemot, Chapter 7), the response was that such magical feats were unimpressive in Egypt, given how ubiquitous such sorcery was in that country. In fact, the Pharaoh’s sorcerers responded in kind. According to the midrash, Yohana and Mamre – the Pharaoh’s chief sorcerers – taunted Moshe by saying “Why are you bringing grain to Aforayim?” i.e. why bring something to a place where it is commonplace? Why bring magic to Egypt, which is overflowing with such sorcery? In response Moshe said to them that it is a common expression that one should take his vegetables to the place of vegetables. That is to say, if someone wants to sell vegetables, the best place to do that is the place where vegetables are found, since that is where the buyers will come. Similarly, the place where the awesomeness of the Israelite God will be appreciated through magic, is specifically in Egypt where sorcery is commonplace.
In Midrash Rabbah (Bereishit Rabbah 76:5) the Sages relate a similar story regarding Yosef, who was seen performing miracles in the house of his Egyptian master, Potiphar. From the text in Bereishit Rabbah it appears that this expression is one of a list of examples of things that are commonplace in a given locale and are, therefore, inappropriate to bring there. Aside from grain to Aforayim the list includes bringing earthenware pots to Kfar Hanina (apparently there was excellent and plentiful clay there), bringing wool to Damascus (where flocks of sheep were common) and sorcery to Egypt.