It is said in a narration from the prophet that trading in things that are unlawful is forbidden. What about if we sell pork/swine flesh to a person who is not Muslim? Is it also forbidden?
Trading things that are unlawful is also unlawful.
Allah the Almighty commands:
“Whoever intercedes in a good cause shall have a share from it; and whoever intercedes in an evil cause shall have a burden from it. Allah always ensures the recompense of everything.” (4:85)
It is narrated that Prophet Muhammad said:
“Surely Allah has forbidden the intoxicants and the money earned from it. He has forbidden the carrion and the money earned from it. He has forbidden the swine and the money earned from it.” (Abu Dawud, Commerce, 66/3485)
“Goddamn the Jews! Allah had forbidden for them the tallow of carrion. They sold it and earned from it. But when Allah forbids something to eat, He also forbids the money earned from it.“ (Abu Dawud, Commerce, 66/3488)
Anas ibn Malik said:
“The Messenger of God condemned ten people about intoxicants: the producer, the supplier, the transporter, the seller, the buyer, the server, the waiter, the offerer, the drinker, the earner.” (Tirmidhi, Commerce, 59)