Everyone who interacts with you is trying to influence you to feel, think, say, or do something. This fact, alone, should never be advanced as a basis for mistrust. Some people have innocent intentions. Some people have not-so innocent intentions. We can’t always be sure what people’s motives are, but there’s one thing we can know for certain: everyone is selling something.
Some people are selling products. Some people are selling services. Some people are selling experiences. Some people are selling ideas. Some people sell what they sell for money. Some people sell what they sell for intangible rewards. Some people sell for the pure joy of selling. Some people sell what they sell because they sincerely have your best interests in mind. Some people sell what they sell without much regard for your well-being at all. But everyone is selling something. The process of engaging in the voluntary exchange of ideas, experiences, and things is a natural and unavoidable aspect of being human.
The person who says “never trust a person who’s trying to sell you something” is trying to sell you on a very specific idea about people who sell things. Not only is this bad advice, but it’s also fundamentally anti-human. Should we trust everyone? Absolutely not. But we needn’t think cynically in order to think critically.