Have him meet you at your favorite local bike shop (LBS) and see if he has a bike receipt so that you know that the bike was not stolen.
Have your LBS see if that size fits you.
Pay your LBS to look for bike problems, ie. frame cracks, old parts.
Pay your LBS to do a bike fitting
That's what I would do if I were to buy a bike from Craigslist
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2012 Specialized Amira Elite, upgraded carbon handle bars, Jett saddle 143mm switched to 145mm 2012 Selle Italia Max SLR Gel Flow saddle
2011 Specialized Ariel Sport,suspension post,Serfas Rx Women's Microfiber saddle