Reference: Fataawa al-Lajnah ad-Daa.imah lil-Buhooth al-‘Ilmiyyah wal-Iftaa. v13 p92 q4 of no.7339
Question: Is it permissible for a merchant to earn more than 10% profit on [the sale of his] goods?
Response: How much the merchant earns is not legislated for in the Sharee’ah, however, it is not permissible for a Muslim to deceive those who buy from him, such that he sells at a price unknown (different from the going rate) in the market. It has [however] been legislated that the Muslim should not be excessive in his profits, rather he should be easy-going if he sells and if he buys, as the Prophet ﷺ has encouraged [an] easy-going [approach] in dealings [with one another].
And with Allaah lies all success and may Allaah send prayers and salutations upon our Prophet ﷺ and his family and his companions.
