Hawzah News Agency – If a person wishes to sell something that is impure, he must tell the buyer that it is impure in the event that were he to not tell him, the buyer would be at risk of committing an unlawful act or abandoning an obligatory act; for example, the buyer would use impure water to perform ablution (wuḍūʾ) or ritual bathing (ghusl) and then perform obligatory prayers (ṣalāh); or, he would use the impure item for eating or drinking. Of course, if one knows that telling the buyer would be of no avail – for example, he is someone who is unconcerned about religious matters – then it is not necessary to tell him.

Source: https://www.sistani.org/english/book/48/