Upload & Sell: Off
I could be convinced that the larger the company, the harder it is to implement a real time inventory system. Plus, if you read the link that Helen provided, although it doesn't say so specifically, Adorama appears to have taken a large financial hit in their previous attempt. If so, that's not an excuse but it can certainly affect the planning for a system in the future.
First the disclaimer - I'm not advocating for Adorama (or any other store). I'm simply attempting to demonstrate the complexity of order processing and warehouse management computer systems.
At my day job, I have been part of teams that have installed order processing, warehouse management, and financial computer systems. The larger the company and the more varied the sales methods, the more difficult it is to tell a customer if a particular product is in stock. At one extreme, consider a store with a single employee who only sells only to customers who are physically present. This situation is extremely easy to manage; if the salesperson and the prospective customer can see and touch the item, it is available for purchase. All of the other customers who want that item must wait for the salesperson's attention. The first person to tell the salesperson that he/she wants the item is the one who can buy it.
The other extreme is a large operation that attempts to stock a lot of SKUs (even the ones that don't sell very often) and takes orders through a lot of different channels. Consider a product where, at the beginning of the day, one unit is in stock. Orders can be taken through the internet, from customers who are physically present, via fax, over the phone, etc. If someone in the store is testing the item before purchase, that item is still theoretically available for internet purchase. If I purchase the item over the internet, and shortly thereafter the salesperson in the store sells it to the in-store customer, the item is no longer available to fill my order. It was available when I ordered it, but now it is not available to ship. If a product is selling well, the effect may be magnified since people may be purchasing the item via internet, phone, fax, in-store, etc. all at the same time. This example is just one of the many ways that the computer system can tell you that the product is in-stock but it is not available for a particular order.
Again, I'm not trying to make excuses for anyone, I'm just attempting to shed some light on the complexities of the computer systems and physical availability of product.