You may have already purchased it, but here's my two cents.

Used bikes can be great, but I strongly reccomend them from your LBS rather than online. A LBS has a skilled (hopefully) mechanic that will give it a complete safety check as well as make sure all components are in good working order and will take care of the bike after the sale just as if it were a new bike from them. Online you may be getting a bike that is not in great shape and the repairs can cost you dearly after the purchase.
Not to mention the fit as mentioned above, that is so important!