What is the ruling on consuming protein supplements designated for bodybuilding, given that they contain ingredients such as gelatin and rennet which have undergone processing, and is it considered doubtful, and is it permissible to sell it if one is unable to consume it?

It is permissible to consume and sell protein if its rennet and gelatin are from an animal that was lawfully slaughtered and permissible to eat, or from a living animal, or if its ingredients are derived from non-meat sources. However, if they are from an impure animal, or one that is not permissible to eat, or from a dead animal, then it is not permissible unless the substance has undergone a complete transformation (istihala tammah), which is the view of many scholars. Nevertheless, it is still more scrupulous for you to avoid it in such a case.