Section 3: An offer and agreement being concluded by one individual:

Firstly: An authorized representative selling to and buying from themselves if the client has abdicated responsibility, neither specifically giving permission for or rejecting that transaction.
It is permitted in general for an authorized representative to sell to themselves if the client has abdicated responsibility, neither specifically granting permission for nor rejecting that transaction, [285] It is better if they have granted permission. If they have not granted permission, then it is not allowed. and this was the view of the Malikis [286] The Malikis allow selling when there is no desire for an increase in price. In other words, if there is no inclination from anyone to raise the price, the agent is allowed to sell it for himself. Alternatively, if the trade is in the presence of the client and he remains silent or gives permission, it is also valid. Mawahib al-Jalil by Hattab (6/13) and Hashiyat al-Dasuqi ?ala al-Sharh al-Kabir (3/387). See also Bidayat al-Mujtahid by Ibn Rushd (2/303) and Al-Qawanin al-Fiqhiyyah by Ibn Juzayy, p. 216. and a view of the Shafi`is [287] Fath al-`Aziz by Rafi`i (11/29). , and it was a transmission related from Imam Ahmad [288] On the condition that it exceeds the amount of its price, or that he appoints someone to sell and becomes one of the buyers. Al-Mubdi` by Burhan al-Din Ibn Muflih (4/246). See also Al-Mughni by Ibn Qudamah (5/85). that was declared the stronger opinion by Ibn `Uthaymin. [289] Ibn `Uthaymin said: “There is another narration that permits it because the purpose of the client has been achieved, which is selling at a fair price. This opinion is more accurate, provided that it does not fall below the market price and the client does not prohibit the agent from making the purchase.” Al-Ta`liq `ala al-Kafi (6/66, 67). 

This is for the following reasons:

(1) Because the client’s objective, which is to sell the item for a fair price, is achieved, and the outcome is similar to if the client had sold to a third party. [290] Al-Mubdi` by Burhan al-Din Ibn Muflih (4/246) and Al-Ta`liq `ala al-Kafi (6/66, 67).
(2) The father has the right to sell his children’s property to himself through parentage, and the same applies to authorized representation. [291] Fath al-`Aziz by Rafi`i (11/29).

Secondly: An authorized representative assumes responsibility of both sides of an agreement on behalf of both parties.

An agreement is valid if an authorized representative assumes responsibility for both parties by acting on behalf of the seller and the buyer at the same time, and this was the position of the Malikis, [292] Al-Taj wa al-Iklil by Mawwaq (4/483) and Sharh al-Zurqani `ala Mukhtasar Khalil (5/297). See also Al-Dhakhirah by al-Qarafi (8/10). Hanbalis, [293] Al-Insaf by Mardawi (5/277, 279), Kashshaf al-Qina` by Bahuti (3/473), and Sharh Muntaha al-Iradat by Bahuti (2/195). a view of the Shafi`is, [294] Al-`Aziz Sharh al-Wajiz by Rafi`i (5/227) and Rawdat al-Talibin by Nawawi (4/306). and the preferred opinion of Ibn `Uthaymin. [295] Ibn `Uthaymin said: “If one person represents both sides of a contract, when does the option [to annul] apply? They say: ‘There is no option because if we say there is an option, then the trade remains permissible, as it is impossible for the person to separate from himself.’ For example, if I appointed you to buy a book for me, and another person appointed you to sell it to him. So, you say: ‘I bought the book from So-and-so for So-and-so.’ Here, the agent has represented both sides of the contract. The correct opinion is that the person representing both sides of the contract has the option, and the determining factor is his departure from the place where the sale was concluded. If the agent says, ‘I bought this book from So-and-so for So-and-so’, and then he gets up and walks away, the trade is now binding.” Al-Sharh al-Mumti` (8/271).

Evidences:
Firstly: From narrations

Abd al-Rahman ibn `Awf narrated that he asked Umm Hakim Bint Qariz, “Will you entrust your matter to me?” “Yes”, she replied. “I hereby marry you off”, he said. [296] Reported by Bukhari in an emphatic form (sighat al-jazm) before hadith no. (5131) and in a mawsul narration by Ibn Sa`d in Al-Tabaqat al-Kubra (10936). The wording here is from Bukhari. Graded sahih by Albani in Irwa’ al-Ghalil (1854). Here, Abd al-Rahman ibn `Awf took responsibility for both sides of a marriage contract, which is weightier than a trade, so it is even more appropriate for this to be permissible in a trade. [297] Nayl al-Awtar (6/267) by Shawkani.

Secondly: An authorized representative in such a situation is given permission for both sides of the contract, so it is permissible for him to fulfil both, as with a father who is permitted to purchase the possessions of their children for themselves. [298] Nayl al-Awtar by Shawkani (6/158).

Thirdly: Because of the absence of any complaint, [299] Al-Mughni by Ibn Qudamah (5/86). for in this context, an authorized representative is not concluding an agreement for himself but rather for two other individuals, and the extent of his contribution to the matter is drafting the agreement on their behalf. [300] Kashshaf al-Qina` by Bahuti (3/473).