It works because the US requires financial instruments (money orders, cashiers checks and such) to be paid out within XX days (10?) so as not to hinder commerce. If the instrument is counterfeit, the cashing institution will charge back the person who cashed it in.
The scammer looks for someone who will think the money order or check is good because the bank pays it in good order. After all, if you were sweating it and the check cashes, it all looks legit. In the mean time, the bank has just released the money according to law, and they pass the money order on down the line for payment.
At some point it will hit the issuing bank and be denied as counterfeit or stolen, and the institution refuses to pay.
When the bank can't cash it after 30 days they want the person who cashed the fake instrument to pay their money back.
This is also a double hit, because the scammer usually says they have someone in the US that they need to send money to anyway, and they will just send say 1000 to you, you keep the $200 owed to you and pass the rest along.
When this happens the victim goes out and uses the funds to buy a legitimate money order and send it to the "friend" of the scammer. This money order will clear without difficulty before the bad one fails.
The giveaways are that the scammer is usually out of the US traveling or working, they buy something and pay extra, and they want you to send the money along after you cash it. They sometimes offer you a portion of the money you pass along as a handling fee.
Stay away if any of that is going on.
You used to not be able to sell laptop in Houston on Craigslist without dealing with these people.