Is it forbidden for an employee to purchase defective devices from customers without the administration's knowledge, repair them, and sell them for his own account?

It is not permissible for a private employee to work for his own benefit during the period of employment, because his services during this time belong to the employer. Thus, the services of an employee during official working hours belong to his company or institution, and it is not permissible for him to work for his own benefit during this time except with the permission of the company owners.