As a customer, if I wanted to know if you have something at a counter, even if I said “are you sure?”, if you clicked 3 times randomly on your computer screen then I’d be sufficiently convinced.
Why do you need to ask if you’re sure? When I worked retail the quicker I got you what you wanted the quicker you would leave me alone so why wouldn’t I be sure? It’s not like I would pretend to check.
If I asked if you had X and you immediately replied No… I would ask to be sure. If you did any token move, pretend or otherwise to check if you have it then I’m fine with it.
Here’s the thing: you might know the answer immediately because you’re an experienced worker but not everyone is trained. Companies spend $0 on training now and customers don’t know who’s the real person to ask.
I don’t ask something rude like “are you sure?”, I say “oh I thought they were on aisle 3?” or something. Then you can see the person really knows where something is.