Honesty in trade is Sunnah. The Prophet ﷺ found wet grain hidden beneath a heap of food and said: “Why did you not put it on top so people could see it? Whoever deceives is not of me.” (Muslim). And: “The two parties to a sale have the choice as long as they have not parted; if they are truthful and disclose, they are blessed in their transaction.” (al-Bukhārī).
العربية: عن أبي هريرة رضي الله عنه أنّ رسول الله ﷺ مرّ على صُبرة طعامٍ فأدخل يده فيها فنالت أصابعه بللًا، فقال: «ما هذا يا صاحب الطعام؟»… «مَن غشّ فليس منّي». (مسلم).
When you sell anything — even a car or phone — disclose its faults honestly; deception is forbidden.