The problem with facebook is that no one is there to police ads and weed out to potential scammers like on forums such as Fauna, here, Kingsnake, The Market Place so there is a lot more potential for possible scams people that comes and go etc.

The bottom line is still the same as a buyer because there are legit people on facebook too, DO your homework, research the seller, ask if people (that you can trust) if they know of the seller, done business with him etc