Is it permissible or forbidden to sell an item—which a person bought from a branch of the authorized dealership where I work—to a third party for 3000, after informing its owner that its price is 2900, in order to take the difference in price?

The work of the questioner as a broker is not permissible except with the knowledge of the one paying the brokerage fee, because it is a type of ju'alah (commission/stipulated payment). It is not permissible to lie to the seller or the buyer regarding the price. However, it is permissible to stipulate a known amount from one or both of them in return for the brokerage or mediation.