Flipping cash is a basic confidence game where someone is told to send money via Western Union in exchange for increasing the amount sent by a huge amount, usually a factor of ten. The person that the money is sent to does not send any money back, keeping the money for themselves. The scam works because it appeals to the people looking to make a profit without actually doing any work.
The obvious question is flipping cash legal, and the answer is that it is not. This obviously involves fraud and breaking a contract; the person accepting the money does have a verbal contract to send money back, after all. However, the con artist is rarely caught due to the anonymous nature of the transaction, which makes it hard to track the transaction. As usual, the best way to avoid the scam is to just ignore it when offered.