Here are some warning signs for scammers and phone numbers to use to report a scammer. I got this from nextdaypets.com: 
 
Buyers who claim to be traveling abroad and offer to have their "agent", "business associate", "friend", or "shipper" complete the transaction. They may offer to pay by Cashiers Check for over the amount. This 'buyer' will claim to be trusting you to return the overage of funds. Banks will often cash these checks then hold you accountable for the funds once the check fails to clear. 
Sellers who insist that you use Western Union or other money transfer services. 
Sellers who reside in distant countries. Sellers that request money to be wired to Romania, Ukraine, Macedonia, Belarus, Pakistan, Russia, Lithuania, Egypt, Lagos, Nigeria, Colombia, Malaysia, Indonesia or Cameroon. tend to have a very high incidence of fraud. Unless you have a lot experience with international orders, you may want to decline to send money to these countries. 
Sellers offering a Dog significantly below market value. This is often a scam to engage many people to send money for this non-exsistent dog. You will never receive the dog. 
Sellers who request that a "shipper" be paid directly or sellers that state the Dog will be delivered directly to your door or destination. All Dogs shipped via an airplane must be picked up at the airport. 
Sellers who state that you must send additional funds to get the Dog through customs. 
Seller insists that you use an online escrow service other than Escrow.com. Before using any escrow service verify the company with the Better Business Bureau to insure the business has a satisfactory record. 
Do NOT trust a seller whom:
1. Uses any techniques as noted in the Common Internet Scams and Warning Signs link above.
2. States they are from Africa, Lagos, Nigeria, Cameroon or London.
3. Insists you use Western Union or other money transfer services. This type of payment offers ZERO protection.
4. Speaks of missionary work or AIDS relief work to build trust.
5. Cannot provide additional 'personalized' photos.  
Who should I notify if I suspect fraud?  
FTC toll free hotline: 877-FTC-HELP (877-382-4357)
FTC online complaint form (
www.ftc.gov)
Internet Fraud Complaint Center (
www.ifccfbi.gov)
Internet Crime Complaint Center (
www.ic3.gov)