Is it permissible for someone who purchases goods for their customers from a specific store, where they receive discounts and guarantees, to buy tools for their own work from the same store, benefiting from these advantages?

If you are buying for yourself from a shop, there is no harm in haggling with the seller and getting a discount. However, if you are an agent for someone else, you are not permitted to benefit from the discount that the shop offers on the goods you purchase for others. Rather, this discount belongs to the principal, because the agent must act in the best interest of his principal.