Is it permissible to sell a car purchased from the "Al-Haraj" market with defects without mentioning them, knowing that defects are not mentioned in this market?

The default rule is that it is forbidden for a seller to conceal defects known to him from the buyer, as this involves deception and fraud. This type of sale is called "bara'ah min al-uyub" (exemption from defects), and the preponderant opinion is that the condition is not binding in it. Therefore, it is not permissible for you to conceal the mentioned defect. Your honesty with the buyer and your disclosure of the defects to him will be a cause for bringing blessings, while the opposite will cause them to be removed.