Is selling swimsuits to a girl who will wear them in front of men at the beach considered unlawful?

A mature girl, or one who has reached the age of desire, is not permitted to swim in front of men, because doing so involves fitna (temptation) by exposing her awra (parts of the body that must be covered), even if the clothing is covering. This is because water reveals the size of her bones, and it usually defines the awra.

As for swimwear that does not fully cover the body, wearing it is undoubtedly forbidden. Therefore, it is not permissible to sell such clothing to someone who will appear in it before those to whom it is unlawful for them to see her. This is because it constitutes aiding in sin. Any clothing that is believed to be used for sin is not permissible to sell to someone who will use it for that purpose.