Shulchan Aruch Y.D. 117 discusses the prohibition against doing business involving non-kosher food.  Among the practical points noted there and in the Poskim are as follows:

• The reason for the halachic prohibition is that if a Jew does business involving non-kosher food, he may inadvertently come to eat it. 

Therefore, one is only restricted from selling non-kosher food (Shulchan Aruch), but it is permitted to sell pets that are from non-kosher species (e.g. gerbils, dogs) (Shach 117:1) since those animals are not usually eaten. 
Furthermore, Iggeros Moshe (Y.D. II:37) rules that one may sell non-kosher pet food (assuming it is not issurei hana’ah – see Sappirim 7) even if technically the food is edible to humans, because there is no concern that someone will eat pet food.

• Nonetheless, there are cases in which it is forbidden to do business involving non-kosher food even if it appears unlikely that the Jew will eat the food. 

For example, one may not sell non-kosher food even if it is in a can or some other container that makes it somewhat difficult to access (Iggeros Moshe ibid.). 
In addition, Responsa Chasam Sofer (Y.D. 104-106) says that the issur is structured to apply any time a Jew takes possession of non-kosher food for business purposes even if he never actually sees or comes into contact with the food (and therefore cannot possibly eat it).  As such, Chasam Sofer’s assumption is that a Jew may not serve as a broker for non-kosher food items, except in cases where he never takes halachic possession of the goods; the details of when that does and does not apply are quite intricate, and specific situations should be discussed with a Rav. 
A Jewish-owned supermarket may not sell even the few non-kosher items necessary to round out their selection and attraction for non-Jewish customers (Iggeros Moshe Y.D. II:38, but see Aruch HaShulchan 117:27).
Lastly, one may not buy non-kosher food specifically for his non-Jewish employees (Rema 117:1; see also Shulchan Aruch O.C. 450:6 and Mishnah Berurah ad loc.).

• One is only forbidden from trading in food which is assur mid’oraisah (Shulchan Aruch 117:1). 

Therefore, it is permitted for a Jew to own a non-kosher cheese company (that uses kosher rennet and cultures) since gevinas akum is only an issur d’rabannan.
The Acharonim debate whether one is forbidden from selling infested vegetables (see Pri Megadim Y.D. (S.D.) 84:18), with the lenient opinion arguing that the person is selling (kosher) vegetables and there is no “transaction” on the bugs.  One could possibly argue that even the strict opinion is limited to the very few cases where the infested vegetables are forbidden mid’oraisah.¹  However, it is irresponsible to sell vegetables which most people are not aware are infested, or when most people do not know how to check for bugs, and/or cannot realistically clean; this is especially true in a kosher-certified supermarket where consumers trust that everything is kosher.  Due to these concerns, the cRc does not allow kosher-certified supermarkets to sell frozen spinach, fresh or frozen broccoli or cauliflower (unless it bears acceptable certification).  Furthermore, other vegetables requiring checking are only sold when bearing a sticker which reads “Caution, must be washed and checked for insects prior to use”.

• If a Jew unexpectedly received non-kosher food, he may sell it. 

For example, chailev, nevailos, and teraifos found or created as part of a kosher shechitah, may be sold to non-Jews (Shulchan Aruch).