One line decides almost everything
Zakat falls on wealth that turns over, not on the means of turning it. Stock bought to sell is zakatable. The shelves it sits on are not. The van that delivers it is not. The premises are not, however much the lease is worth.
This is the same rule that leaves a family home outside zakat however valuable, and a carpenter’s tools outside it however good. A thing you use to produce wealth is not itself the wealth. Almost every question a shopkeeper has about zakat is answered by deciding which side of that line something falls on.
Stock is valued at what it would sell for
Not at cost, and not at the price you hope to get once the season turns. What the goods would fetch on your zakat date, in the market you actually sell into. Stock that has lost value is valued at the lower figure; stock nobody wants at full price is worth what it will fetch.
Raw materials and work in progress count too, for the same reason finished stock does: they were bought to be sold, and being part-way through that on one particular day does not move them out of it.
Debts, in both directions
An invoice you expect to collect is wealth you hold, and it counts. An invoice you may never collect is where the positions part — the common one leaves it out until the money arrives and pays for that year, the cautious one counts it now. Both are held; the page asks which you follow.
On the other side, only liabilities falling due now come off. Deducting the whole balance of a long loan would leave a business with full shelves owing nothing, which is not what anyone means by the rule.
A company does not owe zakat
People do. A partnership or a limited company is not a person with an obligation of worship, so what each owner does is take their share of the business’s net zakatable wealth, add their own savings and gold to it, and measure that total against the nisab.
Two partners with equal shares can therefore owe different amounts, and one can owe nothing while the other owes plenty — because the threshold is met by the person, not by the shop.