Sheikh `Abd al-Rahmân al-`Ajlân, lecturer at the Grand Mosque in Mecca
You must consider what is retained in your possession for a full year. After a full year, you must pay Zakâh on the monetary value of the wealth that is for sale. This includes all the merchandise at the retail outlet.
You pay Zakâh on the principle investment in those goods as well as on the the profit earned from their sale. This is accomplished by looking, after the completion of a full year, at what is in your possession of wealth from all your commercial goods that are subject to being bought and sold and then assessing its value along with the value of the money that you have retained for a year.
The Zakâh is 2.5% of this total value.
As for the physical structures and other property associated with the business that are not for sale, like buildings, fixtures, utilities, containers, vehicles, and display units, these are not subject to Zakâh.
And Allah knows best.
Source: Islam Today