His friends ask him to buy computers for them and he adds a profit for himself
Question

                I am a computer and network engineer, and I work with several companies and individuals, buying computers and accessories for them. Sometimes customers come to me, and they may be some of my acquaintances, wanting to buy computers and accessories etc. I tell them how much the equipment will cost them and I include my commission in the price without them knowing that I have added the commission. For example, a computer may cost 2000 riyals, but I tell them that it will cost them 2500 riyals; I buy the computer with my own money then I receive 2500 riyals from the customer or the person after they receive the computer. My question is: is this transaction valid, in which I take commission from them without them realising? Please note that I deal with the companies that sell the computers and the invoice from the company shows 2500 riyals when the price is really 2000 riyals. Please note that I state the specifications of the computer and the time of delivery, and I deal with the customer on the basis of sincerity. What is the Islamic ruling on that?.
Praise be to Allah.
    It is 
    permissible for you to buy the computers that the customers want, then to 
    sell them to them with an appropriate profit, on condition that you take 
    possession of the computer first, before you sell it to the customer, 
    because the Prophet (blessings and peace of Allah be upon him) forbade 
    selling that which one does not possess. 

    An-Nasaa’i 
    (4613), Abu Dawood (3503) and at-Tirmidhi (1232) narrated that Hakeem ibn 
    Hizaam said: I asked the Prophet (blessings and peace of Allah be upon him): 
    O Messenger of Allah, sometimes a man comes to me and asks me to sell him 
    something that I do not have, so I sell it to him then I buy it for him from 
    the market. He said: “Do not sell that which you do not possess.”

    The hadeeth 
    was classed as saheeh by al-Albaani in Saheeh an-Nasaa’i. 

    The customer 
    should also be aware that you are selling and you are not just volunteering 
    to look for a computer for him; but it is not stipulated that he should know 
    the amount of your profit. Rather if he describes the specifications that he 
    wants, you should tell him that you will sell this computer to him, if you 
    bring it for him, for such and such an amount. 

    But if you 
    have volunteered to look for a computer and buy it for someone, then you do 
    not have the right to increase the price of the computer. 

    Shaykh Ibn 
    ‘Uthaymeen (may Allah have mercy on him) said: 

    If an 
    individual has set himself up to do work, then someone comes to him and 
    gives him fabric and says: Sew a garment for me, then he has the right to 
    take payment for that, because he set himself up to do work. 

    But if he did 
    not set himself up to do work, then he does not have the right to take 
    anything, because the other person gave it to him on the basis that he was 
    doing him a favour. 

    So anyone who 
    does the job for another person without a contract is not entitled to any 
    payment, except in three cases: 

    1.
    
    Saving the property of one who is protected in sharee‘ah from destruction

    2.
    
    returning a runaway slave

    3.
    if a 
    person has set himself up for work.

    End quote from
    ash-Sharh al-Mumti‘, 10/88 

    There are two 
    other cases in which it is permissible:

    1.
    If 
    you are not the seller; rather you are an agent working for a fixed wage. In 
    that case you should tell the customer that you will buy the computer for 
    him for a better price than can be found in the market, and that in addition 
    to that you are entitled to a specific amount in return for your research 
    and meeting the specifications that he wants;

    2.
    if 
    you are an agent working on commission, such as if you will receive 10%, for 
    example, of the price of the computer.

    In these two 
    cases you do not have the right to take anything more than that which has 
    been agreed upon, and any discounts or free gifts that come with the 
    computer belong to the one who appointed you to buy it, and you should not 
    take anything of them. 

    For more 
    information please see the answer to question no.
    36573 and
    45726

    And Allah 
    knows best.