The gemara derives from the pasuk, “Vesheketz yiheyu lachem,” that one may sell non-kosher species (and some other forbidden foods) that have come into his possession, but he may not make efforts to acquire the foods in order to sell them. Be aware that this prohibition of purposely acquiring the non-kosher food applies only to food that is forbidden from the Torah (as opposed to rabbinically).[1] Primarily, meat and fish products are likely to fit into this category, although the matter is more complicated than we can address in our context.
According to most Rishonim, the prohibition of trading in forbidden foods is on a Torah level.[2] The Rashba[3] says that the reason is to minimize the possibility of eating forbidden foods; others say it is a gezeirat hakatuv (heavenly decree without a known reason).
The consensus of poskim is that this prohibition applies as long as a Jew owns the food, even if he is not expected to come in direct contact with the food.[4] It is debatable whether holding a small amount of stocks is considered partial ownership of a company and would be forbidden in this regard.[5]
As mentioned, it is prohibited only to purposely obtain these foods. If one “chances upon them,” he is allowed to sell them. There are many complicated questions regarding the distinction between purposely and unintentionally obtaining. For example, what is the halacha concerning buying mutual funds, when it is known that some percentage of the stocks that will be acquired on his behalf are of companies that deal with prohibited foods? In the case of mutual funds, we can be lenient for a combination of reasons.[6] However, it is more problematic to directly buy stocks of, for example, McDonald’s. One is not allowed to be a merchant of prohibited foods if he actually owns the food. In a case where most of the food is permitted, but business circumstances require the owner to include some non-kosher food, there is some room for leniency.[7] This is not the case, though, when buying stock in McDonald’s, which conducts most of its business with non-kosher food. A partnership with a non-Jew does not help.
If you have a specific question, please let us know. Often, small details can make a big difference, and this summary is not meant as a p’sak for a specific case.