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.
And Allah knows best.