Since Leanne was frequently glancing up as she re-stocked, to make ensure any customers who came to the counter wouldn't be left waiting for long, the rude remark immediately irritated her even before she realised who was speaking. She didn't know Zabini particularly well, having finished school before he reached prominence as Head Boy, but she'd noticed from journal entries that he seemed to have an inflated idea of his own importance and enjoyed ruffling feelings for his own amusement.
With that in mind, and also because it was part of her personal project to learn not to react without thinking, she exhaled slowly before turning around with a charming smile on her face. One way she was trying to deal with rude customers was to act like the girls she'd seen in shops in Japan, where the customer was treated as king, no matter how rude he was. "What can I get for you, Mr Zabini?" she didn't bother pointing out he'd only been standing there for a couple of seconds because she was sure he was only trying to rile her.