Is it permissible to sell or rent secret cameras designed to monitor domestic workers in the home?

It is permissible to sell anything that has a lawful benefit, even if it could be used for something unlawful, as long as the seller does not know that the specific buyer will use it for something prohibited. The basic rule is that it is not permissible to spy on domestic workers unless there is a strong need due to their behavior. However, monitoring them with their knowledge to prevent them from shirking their duties is permissible, and this is a lawful benefit. Therefore, there is no objection to selling these devices unless one knows that they will be used for something unlawful.