Is it permissible for a sales representative to purchase products not available at their company, which management has refused to provide, and then sell them for their own account without the company's knowledge?

It is permissible to buy goods and sell them to whoever requests them under two conditions: that this work be outside your working hours at the company, and that the company grants you permission for this work and does not prohibit it, especially if the buyer assumes that you are doing so on behalf of the company. Fulfilling conditions is obligatory, and the company's prohibition of this has a legitimate reason due to the potential harms that may result, such as causing damage to the company and casting suspicion upon the employee.