As you can see there are many ways someone can be scammed. Which is why you should always be careful.
Scams are very common if you are not careful. If you know how to spot them. Lots of times scammers pretend to be someone else like the IRS saying you owe money, or even someone traveling and they need money for a cab. Another way is on eBay you can buy stuff and never get it. It's always wise to see who you're sending money too especially using credit cards.
If you have been scammed contact your credit card company and most times you can get you money back. If you know who scammed you contacted the authorities. In some events the scammed has gotten away with it, and sadly the person who was scammed never got the money back.
http://www.naturalnews.com/051571_statin_drug_scam_heart_surgeon_medical_establishment.html This articles explains how even the most trusted doctors can scam you.
I hope you found this useful and I hope you don't get scammed!