Dr. Muzammil H. Siddiqi, former President of the Islamic Society of North America, states: It is Haram for a Muslim to sell something that is Haram for him to eat or use. The Prophet (peace and blessings be upon him) said, “If Allah makes Haram the consumption of something, He also makes Haram its transaction.” (Reported by Ahmad) Islam is a consistent religion. When it says that something is bad and forbidden, then it wants Muslims to avoid it for themselves and for others. If a Muslim sells Haram things, he is not only inconsistent in his behaviour; he is also ridiculing the law of Allah. He does not care what is right and what is wrong. It is right if other Muslims avoid business with this person. This will be a good reminder for this person to come back to the rules of Allah.
As far as non-Muslims are concerned, you cannot impose the rules of the Shari`ah on them. If you have to buy things from their stores and eat at their restaurant, you make sure that you buy Halal things and eat Halal food. Of course it is always good that Muslims patronize Halal businesses and try their best to do business with those merchants that observe the rules of Shari`ah.