I think there’s a balance to it. You have to meet the customer, and also understand your boundaries. I’ve sold non-existent features that I was unsure if I could deliver, but always solved it since I’m good at smooth talking. I’ll make sure what I deliver brings value, and keep a dialogue with the client. Sales relations are rarely static from my experience.
If you don’t see the value of your own word, I can’t give it back to you.