Adding an inventory or in stock feature is not too difficult to implement with the User Interface (UI), the real work is the maintenance of online counts including accuracy and related business procedures. The following questions should demonstrate how the simple feature becomes a little more complex...
Q1) How do you manage your inventory right now?
Q2) How many items/products do you have to manage?
Q3) How many items are on each order (daily average)?
Q4) How many orders do you process? (daily average)
For example, if you had 100's of items and processed a couple orders per day, you could spend a few minutes manually changing items from active to inactive when you run out of a product. If you handled 10 or more orders, you might use a daily summary to modify product counts in bulk. If you shipped hundreds of items per day, you would probably prefer an automated system that issues inventory as you accept an order (or ship an order) in real time.
The problem domain expands very quickly when you attempt to include related business processes...
Q5) Do you sell products online and via alternate channels?
(mail order, ebay, phone, fax, brick & mortar)
Q6) When does a product become available for sale?
(When it is received, work in progress begins, finished builds, etc.)
Q7) What increases available for sale counts?
(manual adjustment, received product, returns, order cancellations)
Q8) What process triggers inventory replenishment?
(out of stock, product demand, forecasting, SWAG (Scientific Wild Ass Guessing)
The easiest solution may be to mimic the process you already have in place. How to manage everbody's requirements may involve some more serious thinking.
Q) What do you get when you automate a bad system?
A) An automated bad system.