I've actually done that before! It went kinda like this.
Me: I'm sorry, we can't accept a software box that's already been open. Customer: ...Why not? Me: Because the software might have a virus on it an we can't resell it. Customer: ...But why? Me: Because it might have a virus on it. I'm really sorry, ma'am. We can't accept it. *looks at next person in line to come forward* Customer: No, that doesn't make any sense. I bought it so I should be able to return it. Me: I'm sorry, ma'am, it's policy. Customer: Where's your manager? Me: One second. *goes to get manager who says the same thing but I get blamed for being 'rude' anyway*