I bought my Aegis Swift frame and fork on eBay from a small bike shop. I got it for about 1/3 of retail (brand new, but an older model year), and the deal went perfectly. Since the seller was a bike shop, the shipping was professionally done and reasonably priced. But you do have to be very careful, check feedback religiously.
I also sold a nearly new tandem bicycle on eBay. I included tons of pictures and many details. Actually got quite a few bids and got a very good price for it. I had a local bike shop that deals in tandems package it, and I split the cost of that with the buyer (it was higher than usual since it was a tandem). That transaction also went very smoothly, although the buyer never emailed me to tell me that he got the bike, never left feedback, etc. That was a bit odd, but at least he paid!
My brother-in-law bought a bike for our niece on eBay, and although the shipping was slower than he expected, it turned out well also.
I have read that there is more fraud in bicycle sales on eBay than just about any other category (computers possibly excepted). Be very careful out there as there are unscrupulous sellers trying to "sell" stolen bikes or advertising bikes for sale that they don't even have or have any intent of selling. One tip-off is pictures from the manufacturers site rather than their own photos.
Emily
Emily
2011 Jamis Dakar XC "Toto" - Selle Italia Ldy Gel Flow
2007 Trek Pilot 5.0 WSD "Gloria" - Selle Italia Diva Gel Flow
2004 Bike Friday Petite Pocket Crusoe - Selle Italia Diva Gel Flow