If a person wishes to sell something impure, he must tell the buyer that it is impure in the event that were he not to 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 unconcerned about religious matters – then it is not necessary to tell him.