pearlsofislam:

Question: Is it permissible to sell things in the masjid?

Answer: Buying and selling is not permissible in the masjid because the masaajid are built for the remembrance of Allaah as He subhanahu wa ta’ala says:

يَا أَيُّهَا الَّذِينَ آمَنُوا إِذَا نُودِي لِلصَّلَاةِ مِن يَوْمِ الْجُمُعَةِ فَاسْعَوْا إِلَى ذِكْرِ اللَّهِ وَذَرُوا الْبَيْعَ

“O you who believe (Muslims)! When the call is proclaimed for the Salaat (prayer) on Friday (Jumu’ah prayer), come to the remembrance of Allaah [Jumu’ah religious talk (Khutbah) and Salaat (prayer)] and leave off business (and every other thing).”

(Surah Al-Jumu’ah, 62:9)

They (the Sahaabah) used to buy/sell outside the masjid and when he  heard a man crying out in the mosque about something he had lost, he  said: “May Allah not restore it to you.” [1] So all that is related to this matter should not occur in the masjid.

The Shaykh was further asked about buying and selling in the masjid if the intent behind the trade is to benefit the masjid. He replied by saying that even if it is for the benefit of the masjid it is not permissible to buy, sell and do trade in the masaajid. The exchange of money from hand to hand should be done outside the masjid.

— Shaykh Abdullaah Al-Ghudyaan (rahimahullah)

1. Tayseer al-Kareem ar-Rahmaan fee Tafseer Kalaamil-Manaan

[Source: From the lecture Methods to Understand & Memorize Al-Qur’aan]