People thinking you work at a store, but you don't?

No this is absolutely not unusual. This has happened to me at least 7 times too. Just politely say " I'm sorry sir/ma'am I don't work here." , instead of guessing. In addition, I'm guessing the type of clothing you wear somehow matches the employees there or you have a look that tells people," Hey, you can ask me anything!" I hope this helped! -Kevin