The seller must disclose faults in the item to the buyer
Question

                I want to sell a mobile but it has a fault. Do I have to tell the buyer about this fault (if he does not ask me)?.
Praise be to Allah.The seller must disclose 
    any faults in the item which may make it less desirable and reduce its 
    price. There is abundant shar’i evidence to this effect. It was narrated 
    that Wahb ibn ‘Aamir (may Allaah be pleased with him) said: I heard the 
    Prophet  (peace and blessings of Allaah be upon him) say: “The Muslim is 
    the brother of (another) Muslim. It is not permissible for a Muslim to sell 
    something to his brother in which there is a fault without disclosing it to 
    him.” Narrated by Ahmad and Abu Dawood; classed as saheeh by al-Albaani in
    Irwa’ al-Ghaleel (1321). 
And it was narrated from 
    Abu Hurayrah (may Allaah have mercy on him) that the Prophet  (peace and 
    blessings of Allaah be upon him) passed by a man who was selling some 
    foodstuff. He put his hand in it and found that it was wet. He said: 
    “Whoever deceives us is not one of us.” Narrated by Muslim (101). 
And Allaah knows best.