> Part of Cialdini’s large book-buying audience came because, like me, it wanted to learn how to become less often tricked by salesmen and circumstances. However, as an outcome not sought by Cialdini, who is a profoundly ethical man, a huge number of his books were bought by salesmen who wanted to learn how to become more effective in misleading customers.

(Poor Charlie's Almanack, Charlie Munger)

Yes, the problem is that every scammer and salesman uses these techniques also, and if you've run into a few of them, having a complete stranger approach you with the standard Dale Carnegie playbook immediately sets off alarm bells.