How long has he had it? You'll certainly have to inspect it in person. Just check for the usual cosmetic blemishes, if any, and make sure the keys are working, it boots up properly, the trackpad is responsive, if the screen is all right (no dead pixels, etc), it can charge properly, the ports are all okay. Once you get into the system, also check the battery cycles (under Apple icon > About this Mac > More Info > System Report > Power). The lower the better.
Otherwise, a nearly brand new Air for $1150 is quite a good deal. Save on price and tax.
If you really care about these things, you can investigate if it's a Samsung/LG/Toshiba display and/or SSD (some people really care about these things, but really, either manufacturer is good in terms of the display, and my understanding is that the Samsung SSD is slightly faster at transfer speeds than the Toshiba SSD). Please correct me if I'm wrong, those that know better.
Can anyone think of anything else to look out for?