What is your first clue when an online classified ad does NOT look legitimate?
One of the biggest reason social network marketplaces have become much more popular is because of the relative safety of being able to profile people somewhat before even contacting them. Many of us have been scammed and probably all of us have been spammed after emailing someone through contact forms online and in ads.
Very rarely does it happen here because only members can post ads and membership is approved by a local admin. But it does happen a lot on sites like Craigslist. There are things most of us do to avoid getting scammed.