Basically, on big items (L lenses, etc), if anything seemed fishy, I just dropped it. I'm sure some of the deals I ended up passing up on were legit-- but I know others weren't. That is, if the seller was suddenly out of town or wouldn't talk to me on the phone, etc they I just wrote it off as being a scam.
Yeah, you pass a few good deals, but on the same note, you save yourself a lot of grief.