What is the ruling on selling notebooks that can be used for permissible or impermissible purposes, given the strong presumption that buyers – who are mostly non-Muslims – may use them to write things that contravene Islamic law, such as the Gospel or polytheistic beliefs?

There is no objection to selling notebooks on websites to the general public (Muslims and non-Muslims), as long as the sold item is permissible and is mostly used for permissible purposes; the ruling is based on the predominant use. It is forbidden to sell what is exclusively used for a forbidden purpose, or what is predominantly used for a forbidden purpose, or to sell with the intention of it being used for a forbidden purpose.