A seller must show the defects of the commodity (Part No. 13; Page No. 219) because the Prophet (peace be upon him) said:
Both parties in a business transaction have the right to annul it so long as they have not separated; and if they speak the truth and make everything clear they will be blessed in their transaction; but if they tell a lie and conceal anything the blessing of their transaction will be wiped out.
Even if a seller says the commodity is full of defects, it is not enough until he shows the real defect of the commodity so that the buyer will be aware of the defect. Allah knows best.
May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.