Zakah on shares

Dear Scholars. I have submitted a question before regarding the Zakah on shares and understand that if the shares owned are intended for their revenues then Zakah is payable on the revenues but if the shares are intended for trade, Zakah is payable on the capital and it's revenues.
The shares that I own are intended for their revenues so I only pay Zakah on these revenues but what about when I sell the shares? There are two reasons why I may sell the shares: 1. that I am in need of the money that I would gain by selling them, 2. the market price may be high enough to alert me to the possibility of making a capital gain. Although my intention was to own these shares for their revenues, at any point in time I may find it desirable to sell them if they reach a very high market price. In this case, how would i determine my Zakah, given that this kind of buying and selling may well occur more than once in any year?
Also, do you know of any trustworthy organizations in Qatar that will carry out these Zakah calculations on one's behalf?

Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

Whoever contributes in a company and his intention is to get profit from the annual income/return of its assets and he does not intend to shop in it, then he is not asked to pay the Zakah of the assets themselves. But he should pay the Zakah of the profit if it reaches the Nisaab and if he owns it for a year.
If he needs to sell all these assets or some of them for their increasing value or for some other reason, then he is not asked to pay the Zakah of the assets themselves unless he sells them out. After selling them he should pay the Zakah of the price of the assets that he has sold if it reaches the Nisaab and if a year passes while possessing that amount. Nisaab (minimum amount liable for paying Zakah) is the value of 85 grams of gold. The amount of Zakah due on him is 2.5 percent of all the amount that remains with him until the end of the year which started as soon as he sold the assets.
Allah knows best.