Did you know the internet can be your friend? You can go to say... the Specialized website and it will tell you all about their bikes. Also there is information about what is a CX, AM, DH bike (but I assume you already know that).

Basically do as much research as you can on the internet or by going ot bike shops before you shop either eBay or CL. Be educated about the product and you will be less likely to get ripped... it also helps to know what sort of bike you want. If you are wanting AM, then don't bother looking at DH or CX rigs. Narrow your search criteria but be aware some sellers (ie usually people who have stolen the the bikes) don't know what theya re selling...