Art of the con
I've recently become aware of several of these scams because of things that have happened to my dad. Like the place that offered "free" piano lessons in an attempt to get my dad to buy a $13,000 piano. Or the men who came to my dad's house to fix the cracks in his driveway saying it was $49 a gallon for sealant then telling him they had used 100 gallons. (He of course didn't have the $5k.)
Cons make us feel worse than theft because our participation is part of what makes it work. Worse, our particular contribution is usually a weakness: greed, ego, loneliness, insecurity, ill-health... In addition to the con man + our weakness, urgency is another part of the equation. Con men know that time is not in their favor. We might notice something isn't right. So they create urgency to get us to act.
While cons are usually tailored to context, there are many that have been used for thousands of years. (Wikipedia has a list, if you are curious.)
Living suspicious of everyone doesn't produce a happy life. In fact, a lack of trust can cause us to miss out on some of the best parts. So the cons go on. And the only protection is awareness of our own weakness.