The Ruling on Dealing with an Innovator in Buying and Selling and the Like

Question:
O Shaykh, regarding the case where it is known that this merchant is a Rafidi, and his goods are well-known to people, meaning: goods that are available in the markets, should he be warned against on the basis that they do not buy from him, and it is said to people: do not buy these goods from him; so that they do not support him?

Answer:
This is a matter for consideration. Buying and selling from disbelievers is permissible; the Prophet (peace and blessings be upon him) bought from the Jews, and he died while his armor was pledged to a Jew in exchange for food for his family, (peace and blessings be upon him). However, their belief must be clarified to him; so that he does not take them as friends or companions. As for buying from them if there is a need to buy; the matter is easy. But he must not show them loyalty, nor eat from their slaughter, nor their food; for their slaughter is forbidden.
Q: O Shaykh, is it possible to buy from others?
A: That is preferable. But the intention is to be cautious of exaggeration and affection, or being lenient with them, or facilitating their deeds and being lenient in them. And it must be clarified to people their disbelief and misguidance, and that these are among their deeds: they curse al-Siddiq, they curse 'Umar, they curse the Companions, they call for aid from the People of the House, and they call for aid from 'Ali. This is major shirk. Cursing the Companions is independent shirk, meaning: accusing them of hypocrisy, and that they are not worthy to be narrated from.