What is the ruling on working as an online marketer for a brokerage firm that acts as an intermediary between banks and clients to facilitate the purchase of cars or real estate or the completion of construction, knowing that the company takes a 2.5% commission from the deal as a service fee?

There is no objection to engaging in marketing for permissible (halal (definition)) things. However, it is not permissible to market for forbidden (haram (definition)) things, nor to earn from them, based on the Almighty's saying: "And do not cooperate in sin and aggression." And "the means have the rulings of their ends." Especially if the transaction involves an interest-based (ribawi) contract, for the one who consumes usury, its giver, its scribe, and its two witnesses were cursed, so "they are equal." Al-Nawawi says: "This includes the prohibition of assisting in falsehood," and Ibn Taymiyyah says: "If a man assists in disobeying Allah, he is sinful." Therefore, brokerage is permissible as long as it does not involve assisting in the sale of something forbidden or guiding to what is forbidden to sell.