Not just with bikes, but parts and such too, I always ask why they're selling. I ask before meeting up, but can see the benefit of on-site. For me if I get to the in person stage I ask again, because the consistency isn't always there. It's just amazing what can come out of that question. I've walked away a couple times because a good deal just isn't worth contributing to the stolen bike scourge if I can help it.

I second the bikeregistry website, I've used that a couple times. And although I certainly understand why a seller would want to meet me at a neutral place, the couple times I've bought bikes off of CL going to their homes put me at complete ease. In one case the guy was an amateur lowrider framebuilder and it was fun to see his stuff and see that his "why selling" story made complete sense.