Is it permissible to sell a special number to people considered extravagant, or for the purpose of a charitable project, taking into account that buying special numbers is considered extravagance, waste, and a squandering of money?

If the ruling is that purchasing distinctive (vanity) numbers is prohibited because it constitutes forbidden extravagance, then it is also forbidden to sell them to those who request them, even if the seller's intention is to donate the price for charitable causes. Allah is good and accepts only what is good. Good intentions do not justify unlawful means. It is obligatory to advise those who squander their wealth, enjoin them to good, and forbid them from evil, without assaulting their property or taking it unlawfully.