Erm, she sees an eight year-old alone with a shopping trolley full of stuff. That's not normal. I'd be more creeped out if she treat him like any other customer without asking if he was with an adult. If anything she gave up too easy.

I think I once read a trivia note that said the cashier was actually Marv's girlfriend and she was trying to get information about what houses they (he and Harry) should rob. As always, though, I could be mistaken, but that's my understanding of why the cashier was so nosy.

Because she knew he was a dirty liar and was trying to suss him out.