If it's a bad product or bad salesperson, they'll probably end the conversation quickly, because their whole premise is to catch you off guard and convince you to buy something that you never would if you had 30 seconds to search the internet for reviews.
If it's a good product and a good salesperson, they'll gladly answer your questions and give you a business card. Then you can verify their claims later, and you have a somewhat-trusted contact that you can go back to.
It's a quick way to filter, even if you know you may end up purchasing the product within a couple of days.
This tactic is also a way to quickly stop a potential email conversation with a recruiter, while still being able to have them as a connection. "I'm sorry but I'm definitely not changing jobs for at least 6 months, but thank you for reaching out to me," is enough to do this.