Scams on the internet have unfortunately become a very real concern and something every prospective puppy buyer should educate themselves about. Scammers have become very smart and often not only steal photos but the entire ad wording from legitimate ads, so they sound very convincing. Many of these scammers live in other countries and do nothing but post ads all day, they post 100's of them so even though the advertising sites try to take their ads down and many people watch for them and report them it can be a losing battle.
First of all take a good look at the ad, and look at their other ads. Very often they will have one ad each of multiple breeds. Do the photos appear to match? Or does it look like every photo was taken by a different person? Make sure the phone numbers on the ads and the website match, many times they provide a link to the real breeders website hoping you will call the number in the ad instead of the actual breeders number.
If the price seems to good to be true it probably is, if someone is advertising a beautiful young puppy for a few hundred dollars including shipping it is likely to be a scam. Delivery costs a breeder from $200-$300 or more so it is unlikely that a breeder will sell a puppy for basically nothing.
If you are unsure ask the breeder to send you a current photo with some identifying item in it, either a card with your name, or some people will ask to include a can of a specific type of soda or some other object.
Be careful how you send payments. While there are many honest breeders that take payments through bank transfers or wire transfers such as western union, remember that once you have sent your payment with these methods you cannot get your money back if it turns out to be a scam. Scammers will almost always ask for payments with these methods. I personally recommend using paypal or paying in person.
Lastly, if you have a highly suspicion nature or have had a bad experience I highly recommend you find a local breeder so you can visit them and put your mind to rest. While we as breeders are happy to do anything we can to put you at ease that we are legitimate and honest, working with a overly suspicious and un-trusting customer is not a pleasant experience for a breeder and we would rather you find someone you are more comfortable with. We want everyone getting one of our puppies to be 100% comfortable. =)