What are you looking at?

The biggest issue with buying used is finding your size. (For some reason, almost all the used bikes around here are 58cm or so.) I would also bear in mind that manufacturers change the geometry every so often, so the 2012 version may not fit the same as one from a few years back.

I would first subtly try to ascertain whether or not it's actually the seller's bike. I'd hate to be feeding theft rings, though it may not be as big a thing here in Ohio as it is in other places with larger cycling populations. I'd also ask how the bike was stored, whether it had ever been crashed (esp if there's carbon on the bike), how it has been maintained (regularly cleaned, at the very least), and whether anything's been replaced. If it's a high mileage bike (in the thousands) and the chain is still original, bear in mind that you'll have to replace the chain and cassette sooner rather than later.