I have bought a few bikes via Craigslist. I have learned to recognize a few warning signs that help avoid total wastes of time.
1) when they provide a phone number, I will enter that number as a search. If the search pops up dozens of ads for various things, or variations of the same thing, or the same thing in different areas, it is probably a scam.
2) No pictures, no interest. Move on.
3) Ask questions. If the seller won't or can't answer them, move on. Questions to ask include ) How long have you owned the bike? ) who rode the bike? ) when was the last oil change? )
4) If you are looking for a specific model or year, learn how to recognize it from a picture. A while back I was looking for a "big wheel" version of a TT-R125 , and was getting frustrated that I would get out to possibly buy a bike and discover it was a small wheel version. Then a light bulb moment: the big wheel has a rear disk, small wheel drum brake, easy to spot in a picture!
5) If the deal seems too good to be true, it probably is. If you contact the seller and they start giving you a story about the bike is somewhere else, but he will deliver, or anything at all seems just wrong then forget it.
There are a lot of honest people out there trying to sell a good bike for a reasonable cost.
I have also tried cycletrader.com but I have never had a good experience from there. It seems to be mostly dealers and people wanting way too much for their bike. I do know people who have sold bikes on that web site, however, so I know there are some legitimate deals there.
Rod