What is the ruling on selling e-marketing supplies to Christians, given that one does not know what they market, but with a strong presumption that they might use them to market things that are forbidden in our Sharia? And is this sale considered one where it is strongly presumed that the buyer will use the product for something forbidden?

If you do not know what these people are promoting and what exactly they use these supplies and accounts for, then the ruling is based on their predominant use of them.

If their predominant use of them is for unlawful matters, then it is not permissible for you to sell them to them.

However, if their predominant use of them is for permissible matters, then there is no harm in you selling them, unless you know that the buyer will specifically use them for unlawful purposes, or if their predominant use of them is inseparable from an unlawful matter. In such a case, it is not permissible to sell them to them.

It is worth noting that it is not permissible to sell to disbelievers anything that assists them in matters forbidden in our Sharia.