The trick is that the check is for an amount much larger than the agreed upon price of the pet. The scammer then asks the potential victim to return the overpayment, usually through wire transfer, back to the fraudster or a third party.

The victim eventually learns the cashier's check is counterfeit and loses the money he or she was supposed to get for the dog, plus any funds wired to the scammer. If the victim actually sent the dog, he or she won't get it back.

A number of the pet scams reported to the IC3 involved advance-fee or fraudulent check schemes, says April Wall, a research associate with the National White Collar Crime Center.

Nigerian pet scam
How it works: Scammers either run online classified ads or create breeder Web sites offering purebred puppies -- typically English bulldogs or Yorkshire terriers -- either free or at a discounted price.

The story can vary as to why the animal is free or discounted -- the current owner is a missionary who needs to find the puppy a new home due to the terrible weather in its current location; the animal was rescued from a natural disaster and needs a good home, etc.

The scammer will then ask interested buyers to pay for the dog's shipment, down payment, inoculations and any number of other miscellaneous fees. The victims wire money for the dogs but generally only get excuses for the delay. Instead, they're repeatedly asked for more money to cover additional "fees" invented by the scammer.

Greedy scammers will concoct even more fees that the victim needs to pay after the dog has been supposedly shipped.

Unfortunately, once you wire the money, it's gone, says Preszler.

See the accompanying slideshow for an example of this scam.

The bait and switch
How it works: Scammers are selling purebreds, "designer dogs," mutts and even made-up breeds through online classified ads and breeder Web sites. Often what people get are different dogs than the ones requested or puppies that are sickly. Sometimes they don't get anything.

Nearly 20 percent of the complaints received over the last year referred to Internet sales, says Stephanie Shain, the director of outreach for companion animals at The Humane Society of the United States.