I've purchased two used bikes from ebay and was lucky enough to have a very communicative and responsive seller both times.

I also had questions for the seller before I bought the bike (both times), so it did help my decision that the seller seemed like a honest and nice person and not just someone who wanted a quick buck.

I would definitely inquire about whether the seller knows how to properly pack bikes for shipping (or will be having a shop do it), because it doesn't matter what condition the bike is in if it gets effed up during shipping.
For example, the seller of my first ebay bike did not put a fork block in the fork, and one dropout was slightly crushed during shipping. Fortunately, it was a steel fork, and I was able to bend it back into shape with a wrench, which saved both the seller and me the trouble of having to return the bike.

If the seller is actually knowledgeable about bicycles (i.e. has more knowledge about the bicycle than what they copied from bikepedia), I would consider that a big plus and feel more confident in my purchasing decision.