[Selling Paper and Pens to an Oppressor]

Answer (and Allah grants success): There is no impediment. Al-Azhar states: “It is permissible to transact with an oppressor—buying and selling—in what one does not presume to be prohibited.” The Prophet (May Allah bless him and his family and grant them peace) transacted with the Jews in buying and selling; he (May Allah bless him and his family and grant them peace) died while his armor was mortgaged with a Jew for barley, despite their dealing in wine and usury, and likewise did the Companions in his time.
One may also cite by His saying, Exalted is He,: “Allah has permitted sale and forbidden usury” [al-Baqarah:275]. The case in question plainly falls under the generality of sale.
Objection: Though sale is permitted, this would be assisting the oppressor, since he will use those pens and papers for falsehood. Allah said: “Do not cooperate in sin and aggression” [al-Māʾidah:2].
Response: This is not the prohibited cooperation. Proof: the Prophet (May Allah bless him and his family and grant them peace) and the Companions bought and sold with Jews who were engaged in usury and wine and in turning people from Allah’s path.
To clarify further: Allah has allowed the oppressor, the sinner, the believer, and the disbeliever alike to acquire property—including notebooks and pens—while forbidding oppression. When oppression issues from someone, Allah punishes him for oppression, not for owning or purchasing paper and pens. Thus neither seller nor buyer is sinful for the sale; the sin is in the oppression, not in the transaction—there is no injustice in mere buying and selling.
Another evidence: at Ṣiffīn, when ʿAlī (Peace be upon him) men seized the water, he ordered them not to bar Muʿāwiyah’s forces from it. That was not counted as assisting Muʿāwiyah or strengthening him in aggression.
One may also reason from Allah’s creating things and enabling people to benefit from them: that is not aiding those who use creation for sin and transgression leading to injustice and disobedience; were it aiding, Allah would not do it—He does not do ugly acts, nor will them; all His acts are fair.
Yes, scholars exclude selling weapons of war to active belligerents and rebels—that is impermissible. They said, "It is not permissible to sell to them.
In sum, what we have said accords with sound reason. Modern international norms move along these same lines: they forbid selling weapons to an enemy while seeing no objection to other kinds of trade.
Source: Min Thimār al-ʿIlm wa al-Ḥikmah vol.2