Is the seller of clothes sinful if the trousers he sells are worn at home, but he does not know whether they will be worn outside?

It is permissible to sell these trousers as long as they are originally used for a permissible purpose. There is no sin upon the seller if the buyer uses them for a prohibited purpose, unless the seller is certain that the buyer will use them for something forbidden. In that case, selling them is not permissible, based on the statement of Ibn Qudamah: "If the seller knows the buyer's intention for that, either by his words or by specific circumstantial evidence indicating it. But if the matter is probabilistic... then the sale is permissible." The Permanent Committee has issued a fatwa stating that it is not permissible to sell women's adornments if the seller knows that the buyer will use them in a way prohibited by Allah. It is permissible if he knows she will adorn herself for her husband, or if he knows nothing at all.