What is the ruling on taking an extra thousand [currency unit] from the car seller without the buyer's knowledge? And what should I do with the device I bought with this money if the sale is invalid?

Brokerage (samsarah) is permissible as long as it does not involve assisting in the sale of something prohibited. It falls under the category of a stipulated fee (ju'l). Therefore, there is no harm in you taking the aforementioned amount, as it is a stipulated fee from the seller in exchange for finding a buyer for his car. It does not matter that he was prepared to sell it for a lower amount, because he is allowed to sell for whatever price he wishes. The sale with the buyer occurred by mutual consent, based on the Almighty's saying: "O you who have believed, do not consume one another's wealth unjustly but only [in lawful] business by mutual consent."