Did you know that about 30.2 million Americans, 13.5% of the adult population were victims of fraud in 2007 alone?
Here's where the defrauded money went into:

10. Business Opportunities: .8 mil
9. Credit Repair Scams: 1.2 mil
8. Advance-Fee Loans: 1.7 mil
7. Unauthorized Billing - Internet Services: 1.8 mil
6. Credit Card Insurance: 2.1 mil
5. Work-at-Home Programs: 2.4 mil
4. Prize Promotions: 2.7 mil
3. Unauthorized Billing: Buyers Clubs 3.2 mil
2. Foreign Lottery Scams: 3.2 mil
1. Fraudulent Weight-Loss Products: 4.8 mil

How do you protect yourself?
1. Check the seller's antecedents - Google the name and look closely at all search results.
2. Stay away from all those internet ads that feature the anove listed scams.
3. Don't be greedy and lazy - there is no, I repeat, no free lunches on the internet in real world.

source: 2007 Consumer Fraud Survey (PDF) [FTC]