Don't worry about hurting his feelings. I don't know of any seller or breeder who would be hurt over double checking the sex for a buyer. If someone was than that is someone I would not want to do business with anyway. If they can't admit a mistake can happen before the sale imagine how helpful they will be after....
Also, unless you know what your looking, you still are trusting the seller when having them sex it in front of you. I can take a male BP, pretend to pop it, and show you it's a female. It's not hard to fake that, and unscrupulous dealers still do it all the time.
Ask him to show you how to pop it. He should point out the whitish colored scent glands on the female at the edges of the cloaca. A male will pop longer red colored hemipenises that will originate more inward of the cloaca.