Is it permissible to sell a customer's merchandise at a higher price in exchange for a commission for myself, without the knowledge of the company I work for?

Goods are not owned except upon the buyer's possession of them, either by himself or through his agent, due to the hadith of Hakeem ibn Hizam, may Allah be pleased with him: "If you buy a sale, do not sell it until you take possession of it." Accordingly, if the buyer of the goods authorizes you to take possession of them and sell them, and you take possession of them and sell them, there is no harm, provided that this is done outside the company's premises and working hours.