Is it permissible to take a commission for selling a device, then buy the same device?

The agency in selling is a trust (amanah), and the agent must secure the highest price for the benefit of his principal. What you did by purchasing the device for less than its value and taking a commission is a betrayal of the trust and an additional reduction in price. Therefore, you must repent and inform the principal of the truth of the matter. If you wish to purchase the device, inform the principal and agree upon the price. An agent selling to himself without the principal's permission is impermissible because custom dictates that the agent sells to others and secures the highest price, which is not achieved when he sells to himself.