Depends on the place. Not all places have their inventory system hooked up to their online store.
But places that don't have their inventory system hooked up to their online store shouldn't have anything on the website indicating that an item is either "in stock" or "not in stock".
Claiming that an item is in stock is certainly sending the message that their inventory system is hooked up to their online store.
So it may be either gross incompetence or "bait and switch". In any case, it's deceptive.