Bought my Titus Racer X from Ebay used and was very happy with it. Also bought my Aegis Swift frame off ebay and was delighted. In both cases the seller was a small LBS/dealer with great feedback, so I knew the bike wasn't stolen.
You definitely need to know whether the geometry will work for you, and I'd only buy from sellers with a proven track record.
Good luck!
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