It is very common in industries such as fast-moving retail with a large number of SKUs. Not only inventories fluctuate and may have massive seasonal differences, but as a buyer you have a choice whether you want them to stock up their shelves at the time of purchase or run a fire sale to reduce the stock. As the cost of inventory can run very high, your decision matters a lot. Imagine the difference between buying a retail store in January (low season) with reasonably empty shelves that you are going to gradually stock up, vs buying a store in October with full shelves for the holiday season. Ask them for their inventory history and seasonality, and make sure to look into their supply chain (is it stable, do you need new licenses and agreements to be able to resume supplies immediately).

In the past I've discussed with a seller that I'm looking to buy a functioning business, and the inventory is a part of the business running properly. You could extend the "inventory not included" logic to anything in the business. Some sellers get hung up on it in which case you can just kind of back the proper inventory level value out of your offer and not really discuss that explicitly with seller. In some cases the seller is trying to make the point that inventory is above what's needed to run the business, so I'd be open to compensating for that. Of course ideally you would check for expired inventory if possible or make provisions in the docs if seller not comfortable with you being in store prior to close.