Tineye is a photograph database that you can check online photos against. It is not always 100% since photos cycle out of the database based on age, but it is a good part of your process to determine if someone is legit or not.
Here is the website.
http://www.tineye.com/
If you use Firefox, there is a nice plugin available where you can just right click on a pic on a website and check it against the database at Tineye. You can also download the pics and browse to them with the website.
Take this ad on CL:
http://gulfport.craigslist.org/ads/1828019339.html
If you run it through Tineye, you'll see the pic is on 14 different websites across the world. Good bet the ad is a fake, bait and switch, or just plain spam.