Is it permissible for an employee to take goods from the company he works for and sell them in his private store without the knowledge of the company owner, given the latter's refusal of such an action?

Sales require mutual consent between both parties, as Allah Almighty says: "O you who have believed, do not consume one another's wealth unjustly but only [in lawful] trade by mutual consent." Therefore, if the company owner does not wish to sell to you, it is not permissible for you to buy from him, even if you were to buy at the same price or higher, because the fundamental principle in financial transactions is consent.

However, if you are an agent for the company in selling goods, it is not permissible for you to sell to yourself, because an agent does not sell to himself due to the conflict of interest involved, and because the fundamental principle is that the offeror and acceptor should not be one and the same. But it is permissible for you to buy from someone who bought from the company, not directly from the company itself.