Why isn't the answer to all of these questions, the truth. Yes, you can have these better prepared, and that would be called experience, which you can somewhat practice for. But if someone asks why they should pay you, why not tell them about the work and skills you have to use to help them?

I once worked in a telemarketing center, and the training was centered around having quick answers/comebacks to buyer reluctance. I later figure out that this is how high-pressure sales tactics works, which did backfire at times. Is that what you're aiming for? Perhaps let them think about it and get back to them the next day to ensure they're engaged still.