1. The guy trying to sell me something - anything - was not my friend, no matter how friendly he seemed.
2. That verbal promises were meaningless, even without the statute of frauds (anything dealing with real estate has to be in writing). Just because some salesman says something, doesn't make it true.
3. Advertisements are not factual documentaries, but a series of small lies designed to get you to buy things.
4. Anything that seems too-good-to-be-true probably is. When someone tells you that you can "have it all now" and pay later, odds are, it is a shifty deal.
5. The marketplace is an adversarial battleground. Just because the guy at the corner store smiles and is nice to you, doesn't mean that his financial interests aren't often diametrically opposed to yours. Yes, sure, he wants happy return customers. But yes, he also is making a profit from each and every purchase you make.