One thing nice about buying used (if you are sure the bike fits, as everyone has stated!) is that you can get a bike for maybe half or less $ than what you would pay for it new. I recently sold a Terry Isis in very good condition for less than half of what it cost new, and I think the buyer got a great bike for a very good price. I also recently bought a mountain bike off eBay, and even though it had been fairly well used (6 years old) so that I had to replace the wheels, cassette, chain, crankset (for my leg length), saddle to better suit me, put my own pedals on, etc., had an LBS pump the shocks and give it a tune-up and once-over, it still cost less than half of a new one would have run me! Even though I'm sure I'll eventually have to replace the shocks, it was still better than buying new.
Major caveat: This was an expensive model that would cost about $5000 (as currently equipped) new. Buying a relatively inexpensive bike used and having to replace as much as I did would not be cost-effective as you could buy a new one for less. But in your case, if the bike is in essentially new condition, fits you, and is a great deal, go for it!
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