Is it permissible to sell computer programs although I am afraid that they will use them for sinful purposes and that I may incur a burden of sin as a result of that, or am I not responsible for what the purchaser does after he has bought it from me?
With regard to programs and computers that may be used for good or bad purposes, it is permissible to sell them and trade in them, unless it is known that the purchaser is going to use them for haraam purposes. In that case it is not permissible to sell them to him. The same applies if it is thought most likely that he is going to use them for haraam purposes; it is not permissible to sell them to him in that case.
But if you are not certain as to whether he will use it for haraam purposes or not, then there is nothing wrong with you selling it to him. Then if he uses it for haraam purposes, the burden of sin will be on him only.
Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him) said: It is not valid to sell that which is intended for haraam purposes, such as juice that will be used to make wine, if that is known, as is the view of Ahmad and others, or if it is thought most likely, as is one of the two opinions of Ahmad. This is supported by the fact that our companions said: If the landlord thinks it most likely that the renter is renting the property for sinful purposes, such as selling alcohol and the like, then it is not permissible for him to rent that property to him, and the rental contract is not valid. Sales and rentals are the same.
End quote from al-Fataawa al-Kubra, 5/388
See also the answer to question no. 39744
And Allah knows best.