No, that's freedom of speech. But it would be a colossal waste of time.
However if you lied about the person next to you and said anything that was both 1) damaging (in his opinion) and 2) was proven to be untrue about him, then you would be committing "defamation", which he could successfully sue you over, and win lots of money from you.
Most people have better things to do than what you have described. However our country is founded upon freedom of speech. But again, there are limits, you cannot maliciously lie about someone else and expect to get away with it. That is malicious and harmful, and it is treated accordingly by the eyes of civil law. You also cannot shout fire in a movie theater either.
But in general, if you just want to say they shouldn't go with him, thats fine. If you use TRUE FACTS of why they shouldn't go with that salesperson, thats also fine.
Also if the person needs a permit for the booth, then you would need a permit for yours. You'd need a permit even if you stood there talking business without a permit, as you would be competing/harming his business, upon which he certainly could get you removed if you didn't have the necessary permit.
(I don't LIKE permits, but I certainly agree they should be given out fairly, equally, and easily. The laws regarding permits should be equal. If he needs one, then you need one. And if he can easily get one, you should be able to easily get one too.)