I've sold 6 bikes on ebay. I would demand good, up close pictures of the bike before bidding. A lot of stuff won't show up with poor pictures or ones that are too far away to see detail. Ask many questions about every little detail. Such as rust, corrosion, stripped bolts, oil leaks, bent radiators, dented rims, chassis greased, etc. Keep in mind that one guy can view a bike as "in really good shape" another guy can view it as "a total pile of crap". Some ads are written horribly or lack detail. If the guy can't elaborate on it's condition or are short on words be leary. The only exception is if it is new or as new.
The older the bike is, the more chance it has been cannibalized or abused.
Don't even consider a bike without pictures.
Some guys refuse to help with shipping or crating- I don't know how they even expect to sell it. I'd stay away from a guy like that unless you can pick it up yourself.
Check the guys feedback profile.