well, obviously, I dont bust out a park alignment gauge and dissemble the entire bike and check for internal rust. But I think it's smart to eyeball the frame and fork and pull out the seat post and look inside. A friend of mine once came home with a frameset he purchased off CL. Looked fine, but upon closer inspection, the fork was bent from a frontal crash. The seller never said anything about a wrecking it.

I guess my point is, dont take a strangers word -- check for yourself and dont be afraid to walk.