The thing I worry about on eBay is fakes. How do you tell the difference? I suppose only buying from a guy who's selling one or two scopes, which seem to be his used personal scopes would be okay. High volume sellers scare me. Where are they getting those scopes???
I'm the opposite...more dubious of the guy with one or two auctions that are too good to be true.
If it's not from a highly reputable seller, it's gotta have documentation.