I tend to stay with FM forums for used equipment. There rating system puts my mind at ease more than buying a lens from craiglist. I guess once you learn what to look for (dust, oil, aperture blades), meeting someone locally would be a better idea since you can physically look/try the lens.
But as Chris stated, if your on a budget used can be very rewarding. There is some wonderful glass out there collecting dust.
The problem is, the seller is usually trying to unload a 10+ year old lens for $50 less than a new one.
Check focus, lens mount filter threads, zoom and/or MF focus rings. Also take a couple test shots. If it checks out, no problem!