The author states that sometimes the hardest questions are the big opened ended ones like "Why should we buy from you?"
Here is how he suggested you could answer that question. I really like the emphasis on honesty and openness.
Customer: So, tell us this. Why should we buy from you?
You: Honestly, it would be arrogant of me at this point to say why you should buy from us. I'm not even sure you should buy from us, much less why. What I can tell you right now is why certain people buy from us, and why certain people don't buy from us. I can also give you an idea of why others buy from our competitors, and why some don't. That's not bluster. Some customers put a great deal of emphasis on flexibility, others value clarity of contracts. I don't know which you value more. And that's just one small component; I'd be happy to list the issues we need to talk about in order to learn who you should buy from. Nobody is better than everyone else across every single dimension–including us. So it's critical we talk about your various needs to determine who your best supplier is. If we sit down and talk honestly about what's important to you, together we'll learn whether my company is your the best supplier. So let's get to work. Let's start working to figure out who you should buy from.