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

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.