It is not permissible for a dealer to sell auto parts that he does not have in his shop, because that comes under the heading of selling what one does not own and does not have in one’s possession
Question

                I work in an auto parts dealership, and we cannot keep every part in stock. A customer may come to us to buy some parts which we will deliver to his workshop, and he gives us the money, and he may be aware that we did not have some of them in stock at the time of purchase. Then we go and get the missing parts from our neighbour, then we deliver them to his workshop. Is this transaction permissible? If it is not permissible what is the solution?.
Praise be to Allah.
    In the answer to question no.
    169750 we explained that it 
    is not permissible to sell that which you do not have in your possession and 
    that you are not able to guarantee delivery thereof to the customer, unless 
    you buy it, take possession of it, move it to your place and then sell it, 
    because of the hadeeth of Hakeem ibn Hizaam (may Allah be pleased with him) 
    who said: I went to the Messenger of Allah (blessings and peace of Allah be 
    upon him) and said: A man may come to me asking me to sell him something 
    that I do not have. Should I buy it for him from the marketplace then sell 
    it to him? He said: “Do not sell that which you do not possess.”

    Narrated by at-Tirmidhi, 1232; an-Nasaa’i, 
    4613; Abu Dawood, 3503; Ibn Maajah, 2187; Ahmad, 14887. Classed as saheeh by 
    al-Albaani in Irwa’ al-Ghaleel, 1292. 

    Based on that, the way in which you are 
    doing this transaction is not permissible. 

    The solution is to do it in one of the 
    following ways: 

    -1-

    Agreeing on the required parts in terms of 
    their description, quality, number and price in such a way as to dispel 
    ambiguity. Then make an agreement to carry out the transaction when you take 
    possession of the parts, without any party being obliged to buy or sell. If 
    you do not have the parts in your shop, then you can make an agreement 
    between you and the purchaser to buy them, meaning that the purchaser will 
    come and see the parts, and if he finds that they are what he is looking for 
    and agrees to that before handing over the money and taking possession of 
    them, then the transaction will be valid. 

    -2-

    The purchaser can appoint you to act on his 
    behalf in buying the required parts, after describing them to you and 
    telling you the description and quality required, and so on, in return for a 
    payment that you both agree to, as a fee for acting on his behalf. Then when 
    the purchase is completed and the parts have been delivered, you can take 
    the payment that has been agreed upon. Or you could do this for him for 
    free, without any payment, for that particular part, if you wish. 

    -3-

    You could buy the parts that he wants and 
    that are available in your neighbour’s shop from your neighbour, then bring 
    them to your shop, then sell them to the purchaser in a valid transaction. 
    This may be done very quickly. Instead of selling them to the purchaser 
    first, then buying them from your neighbour, you can buy them from your 
    neighbour first, and then sell them to the purchaser. 

    For more information please see the answer 
    to question 160559 

    And Allah knows best.