Volume 3, Book 34, Number 293:
Narrated Hakim bin Hizam (radiallaahu `anhu):
Allah’s Apostle (sallallaahu `alayhi wasallam) said, “The seller and the buyer have the right to keep or return goods as long as they have not parted or till they part; and if both the parties spoke the truth and described the defects and qualities (of the goods), then they would be blessed in their transaction, and if they told lies or hid something, then the blessings of their transaction would be lost.”
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).
I know for a fact that very little people do this. If someone’s out to sell his product, he’d like to hide the defects knowingly in order to get a bigger price than its worth. Only a believer would do this because he knows that these few extra bucks won’t do him much good if he lost Allah’s pleasure because of his unfair dealing.
Jannah is your real business. Strive to crack that deal. ;)