Is it permissible to take a commission from sellers, or to perform some tasks for myself without the employer's knowledge, in exchange for extra work for which I was not compensated, especially since the current intermediaries take many times what I would take?

An employee is not permitted to take a commission for work he has contracted to do with his employer unless the employer gives permission. This is because it is considered a form of bribery and embezzlement by employees, and it leads to the undermining of the company's right to quality goods and fair prices. The mentioned reasons, such as the employee's shyness in requesting additional pay, or other intermediaries taking larger commissions, do not constitute a legitimate justification for taking a commission without the employer's permission.