He gets the product and sells it for his own benefit when the item is not available in the store where he works
Question

                I work in an auto parts store. When a customer comes to me and I do not have what he is looking for, I get it for him from my neighbour for the best price, then I sell it to him and keep the difference for myself.
Praise be to Allah.
      It is not permissible for a worker or employee in a store 
      to sell for his own benefit, whether the product is available in the store 
      or not, because the time at work belongs to the owner of the store. So he 
      has no right to use that time for anything that does not serve the 
      interests of the owner, because according to business custom, this is 
      regarded as a betrayal and cheating the business.. That is because 
      usually, if the worker does not find the product in his store, he will buy 
      it from his neighbour and sell it to the customer for the benefit of the 
      store owner, not for his own benefit. 

    Based on that, what you are doing is contrary to your 
    employment contract and it is cheating and betraying the store owner. 

    Based on this, you have to refrain from doing this action and 
    repent to Allah, may He be exalted, from this haraam earning. You have to 
    work out how much money you acquired in this way and put it in the store, 
    because it belongs to the owner of the store and it does not belong to you. 

    And Allah knows best.