Is it permissible to adopt the fatwa of Fiqh Councils that permit the sale and manufacture of items used for both lawful and unlawful purposes, considering that the sin of using them for unlawful purposes falls upon the buyer, and that the money earned from them is lawful, and that this does not constitute cooperation in sin? And is this considered a slip?

Selling permissible and beneficial things is lawful, and the money earned from it is halal. There is no sin upon the seller if the buyer uses it for something haram, unless the seller knows or strongly suspects that this will be the case; then it is not permissible for him to sell it. There is no blame on someone who acts, regarding matters of differing opinions among scholars, upon one of the scholarly opinions, as long as he is confident in the correctness of that opinion and is not merely following his whims.