Here is my two cents on the issue:
I am living in Paraguay, a country ranked as the second most corrupt in the World (but some say that the first place was lost only because the local government bribed the second to switch places...
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My job is Treasurer of the local ING branch. This group is one of the biggest financial institutions in the World and the second biggest company in the Netherlands.
All this means that I am trained to avoid scams, money laundering and other generally unhealthy practices that appear from time to time.
So far, all internet scams I saw came from Africa. But I wouldn't discard the posibility of a different origin - including this country. Interestingly, some advisors believe that part of the scams are created elsewhere and emailed from Africa to conceal its origin. The logic is that they require some sophistication in the victim's country financial system.
In my opinion, the first rule to avoid scams is self-discipline. Most scams tempt the victim with the possibility of making easy money or with conditions that somehow make them feel they are getting an advantage or additional safety in the deal. If you stick to correct business practices, you are generally safe. If in doubt, just ask. Ask your parents, friends, your bank officer or a lawyer. Don't think you are smart enough, there is always someone smarter.
The second advice applies to any deal, anywhere: know the mechanics of payments in your currency. Each currency has specific rules regarding money transfers, checks, cashiers checks, payment orders, credit cards, etc.
One example is the case that originate this thread: if you deposit a stranger's check in your account, the check can be returned up to 5 years later (!!) and the sum will be debited directly from your account.
Also, the checks could have been stolen (from the mail, for example). After the checks are returned and you are left without money and boat, how would you prove that you were not involved in the original robbery?
Other interesting features of the US financial system is that FED and CHIPS wire transfers can be reversed the same day they are effected (not easy, though). And we all know that credit card payments are too easy to reverse.
Even if you cashed a transfer or money order, there are instances in which you could be required to return the funds.
To make it short, it is very important to know how currency flows in order to reduce the chance that it flows from your pocket.
That said, a comment on being overcautious}: note that one should not assume that everything foreign (or African) is a scam. If we refuse a deal with a foreigner/stranger for no reason, we may be liable for discrimination. I think this also applies to the US and Europe, so be careful three times... and then be careful backwards!
Take care,