If anyone wants to check out interesting customer interaction in a video game, please play Dragon Quest IV (aka Dragon Warrior 4) on the original Nintendo. Chapter 3 focuses on a merchant named Taloon. As Taloon, you go out and get items by fighting monsters, which are the items that stock your shop. Then you stand behind the counter while people come in and browse what you currently have to offer. Sometimes they'll offer you extra money for an item, sometimes they'll offer less (and you can always deny their purchase). Some people will come in, view your items for sale, decide you have nothing they want and leave. It might be interesting in MLBS if sometimes the customers weren't requesting something, but just came to browse through what you already have in stock.
Here's a guide on GameFAQs.com that details some of the gameplay mechanics: https://www.gamefaqs.com/nes/563409-dragon-warrior-iv/faqs/21473