It might be a good idea to post up the item number, or a link to the ebay page. Chances are that plenty of other people (no doubt some from this very forum) will be watching the same bike, so it just comes down to how much you are prepared to bid for it.
If you prefer though you could just say what make/model/year it is if you're just after general advice about the bike, but again chances are that if somebody wanted to find it on ebay then it wouldn't be too hard to do.