Inventory tracking for in-(physical) store purchases only

I have a client who wants to track the number of items sold - but here's the thing - he can only sell these items in their physical store, not online. 

He wants the website to reflect store inventory so locals know what he has in stock and to come Into the store, but online purchases will not be available. 

Is there a way to this up? Even if they're manually entering the inventory update at the end of the day? 

So to clarify, a merchant sells products in their store and, although they don't want to sell online, they want to show stock levels on product pages on their Squarespace site? If that is correct?

Also, which POS system do they use to track their in-store inventory?