His relatives ask him to buy mobile phones for them, so he charges them a little more
Question

I deal in mobile phones and accessories, and other things that are imported or bought locally, but the fact that I am a university student means that I cannot expand my business. So I buy two or three pieces of certain types and sometimes, when there are discounts, so that I can make some money for myself. But my relatives do not know that, and they believe that it is just a hobby, because from a young age I had the hobby of electronics and I love it. They come to me with any questions they have, or when they want to change their mobile phones and buy new phones. So one of them will say to me: I want to buy a phone, and I advise him to buy a particular phone. Then he says to me: Buy it for me, but he does not give me any money until after I buy it for him. I pay for it with my own money, and after that I charge him. This has happened more than once, and I do the following: when I buy the phone with my own money, I increase the price slightly, and I tell him the price, but he believes that my dealing in phones, and buying and selling, is just a hobby, but I sell to him and to others. But as I said above, he does not know that, because I buy and sell only a few pieces, and I do not have a shop. But if one of them gives me the money before I buy the phone, I do not add anything to the price, no matter what it is, even if I got a discount, as I regard it as his right to benefit from it and not my right. What you think about that? Is it stipulated that the transaction should be regarded as wakalah [appointing someone to act as a proxy], so that the item is to be bought with the money of the one who appointed the proxy and not that of the proxy? In other words, for wakalah to be valid, is it stipulated that the money should be paid by the proxy or by the one who appointed him?
Praise be to Allah.Firstly:
If your relative tells you, “Buy a phone for me,” then he is appointing you as a proxy to buy on his behalf. Therefore it is not permissible for you to go and buy it and sell it to him, or to buy it from yourself if you already have it, unless you inform him of that, because the proxy cannot sell to himself or buy from himself.
Ibn Qudamah (may Allah have mercy on him) said: Whoever is appointed as a proxy to buy something, it is not permissible for him to buy from himself… This is the view of ash-Shafa‘i and ashab ar-ra’y."(Al-Mughni  5/237).
Secondly:
If someone appoints you to buy on his behalf but does not give you any money, then this is a combination of two things: borrowing from you and appointing you to buy something on his behalf with this loan.
Based on that, it is not permissible for you to add anything to the price of the phone, because in this case it is adding to the loan.
If someone appoints you to buy on his behalf and gives you money, then this is wakalah only, and whatever discounts or gifts and the like are given, they are for the one who appointed you as his proxy.
It says in Matalib Uli an-Nuha (3/132): If the seller gives a gift to the proxy who bought from him, it is to be regarded like a discount on the price, so it is part of the deal, because it belongs to the one who appointed him as his proxy. End quote.
It says in Fatawa al-Lajnah ad-Da’imah (14/275): The Muslim must be honest in his dealings, and it is not permissible for him to tell lies or take people’s wealth unlawfully. That includes one whose brother in faith appoints him to buy something on his behalf. It is not permissible for him to take anything from him in addition to the price for which he bought the item. Moreover, it is not permissible for the one who sells it to him to write a different price – other than the real price – on the invoice to deceive the one who appointed him as his proxy so that he can charge him more than the real value, so that the proxy can keep the extra amount. That is because this comes under the heading of cooperating in sin and transgression and consuming people’s wealth unlawfully. It is not permissible to take the wealth of a Muslim unless he gives it willingly. End quote.
Thus you know that it is not permissible to take any wealth except with the knowledge of the one who appointed you as a proxy, and that could be in two scenarios:

If it is given as a fee for acting as his proxy.
If you sell the phone to him, subject to the condition that you take possession of it first, or he pays you in advance (salam transaction), then you go and buy it for him (and deliver it to him later). In either case he should know that you are the seller and he is buying from you something that you own, and he should not think that he is taking from you a phone that he appointed you to buy as his proxy, because one of the conditions of a sale being valid is the consent of both parties.

See also the answer to question no. 87782 .
And Allah knows best.