Do not think that these activities are rare so they won't affect you. Equally they are not a figment of the collective imagination of the internet community. These scams are real and hit the pockets and bank accounts of someone just like you.
The fiscal transfer scam has two forms:
These scams have been used to fraudulently obtain money via both Western Union money transfers and standard personal cheque payments. Typically, you would be approached with an offer for the asking price of an item that you have offered for sale.
The counterfeit payment fraud uses the small print in the policies of banks (certainly the big 5), that the advice they give you of the current balance in your account may not be the actual cleared balance. What they are actually informing you is the balance to their best knowledge: your previous balance plus the cheque you deposited / transfer you received. Only when the money transfer is refused is the payment withdrawn or the cheque determined to be counterfeit. The bank can nullify the payment at a considerably later date.
The forward balance scam can leave you considerably worse off, financially. The buyer asks if you would accept payment from a third party. The payment would be an overpayment as they are owed money by the third party. They request that you forward the balance once the money appears in your account. Once you have forwarded the balance of the transaction the payment is refused as per the counterfeit payment scam.
Often they are international operations so the best defence is to beware overseas buyers or buyers who will arrange for an agent to collect the item on their behalf. Use payment protection or an Escrow service. An Escrow is where an intermediate company takes receipt of the money until the item is received by the buyer, a go between if you like. Never agree to forward on a balance of payment - if you don't fall victim to the scam yourself, then you probably just became involved in money laundering.
Remember, There are many internet scams and if a deal seems "too good to be true," it probably is.