How is the zakat on trade goods calculated—is it based on the purchase price or the selling price—and is what befell her car due to an error in calculating zakat?

The zakat (definition) on trade goods is calculated based on the market value of the goods at the time the zakat is due, not their purchase price. Whoever made an error in this regard must calculate the amount of zakat that was not paid and hasten to pay it, as it is a debt upon them. If they are unable to determine the exact amount, they should make an earnest effort to ascertain it and pay what will certainly or most likely absolve their responsibility. Repentance and seeking forgiveness are due for any negligence in learning the rulings of zakat. As for the incident that occurred, it is hoped that it will be an expiation, and we ask Allah to reward them for their affliction and compensate them with good.