You aren’t wrong. 
But in arms-length commercial transactions, the parties are by definition adversarial.
You can either arm yourself with knowledge and do the required research, or you can take your chances.
This is why we breathlessly encourage people to solicit multiple offers to determine the value of their car in advance, which takes just minutes.
This takes more effort than many want to expend, but that’s a conscious choice. It doesn’t make the other party a criminal for acting in their own self interest, which all of us do all day, every day.