Is it permissible for an employee who works in a computer maintenance shop to buy spare parts with his own money and then sell them to the employer at a higher price without informing him, knowing that this is the price the employer would have paid had he found them?

If purchasing items is not part of your job responsibilities or at the request of the shop owner, then there is no harm in selling them to him at a profit after acquiring and possessing them. However, if the purchase is part of your job responsibilities or at his request, then you are his agent, and it is not permissible for you to sell them to him without his knowledge and consent, because an agent may not sell to his principal from what he himself owns except with the principal's permission.