How should he pay zakaah on a car if it stayed in the showroom for three years and its price has fallen?
Question

                Someone bought a car for 13,000 riyals with the intention of offering it for sale and making a profit. But after he bought it, he found out that it was faulty, which lessened its value, and he left it in a car showroom, where it stayed for three years, and offers were made on it that were much less than the purchase price during that time. After three years, he sold it for 8,000 riyals. Was there any zakaah due on this car that was offered for sale for three years every time one year passed? How should he pay it? What is the rate of zakaah?.
Praise be to Allah.If a person buys a car with 
    the intention of selling it and making a profit, he must pay zakaah on it 
    every time a year passes. He should work out its value on that day, i.e., 
    its market value on the day when the year has passed, and pay one-quarter of 
    one-tenth (2.5%), whether the value has increased or decreased. 
Shaykh Ibn ‘Uthaymeen (may 
    Allaah have mercy on him) was asked: Someone bought some land with the 
    intention of dealing in it, but it remained in his possession for a long 
    time. Is zakaah due on it? 
He replied: 
If a person buys land with 
    the intention of dealing in it, then he must pay zakaah on it every year, 
    whether its value increases or decreases, and whether it sells or not. He 
    should work out its value every year, then if he has money he may pay the 
    zakaah from the money he has; if he does not have money he should keep a 
    record of the zakaah that it due every year, year by year, and when  he 
    sells it he must pay zakaah on the past period. End quote from Liqa’ 
    al-Baab il-Maftooh (15/12). 
Based on this, the owner of 
    the car should have looked at its price at the end of every year and paid 
    zakaah on it. If he did not have any money, he should have kept a record of 
    it and paid it when he sold the car. 
It should be noted that the 
    year with regard to trade goods is the year from when one acquired the 
    wealth with which he bought them, if it reached the nisaab (minimum 
    threshold). The year for the car in this case does not start from the time 
    when he bought it, but from the time when he acquired the money with which 
    he bought it. 
What the owner of the car 
    must do now is work out what the value of the car was at the end of each 
    year, with the help of those who have experience in the market, then pay the 
    zakaah. 
And Allaah knows best.