Fundamentally, if you are an overbearing, self-centered talker, then you are making a huge mistake. If you talk over, interrupt or upstage your customer, you are making a huge mistake. If you are pretentious, you will fail as a salesperson. If you one-up customers, you are asking for the door.
One-upsmanship is a weak personality trait of someone begging for approval. We've all encountered people who can't wait for someone to finish their story about having dinner with a senator, so they can relate their own story about sharing breakfast with a cabinet member. You can see the anxiety as the one-upper taps his finger or rhythmically kicks his leg to hurry the buyer along so he can tell his story. It's lame, and it contributes nothing to a salesperson's objectives to gain a customer and make a sale.