Hi Hershey - KEH is indeed very reputable. In fact, they are probably the best used photography vendor in the world. They almost always understates the condition of the used stock they sell. That being said, they can't possibly check every function on every camera they sell, so it's wise to do the tests that Steve suggested, along with just getting out and shooting like crazy with the camera under as many different operating conditions as you can. Particularly, try all the different modes, eg, Aperture and Shutter Priority, and Manual. If it doesn't behave as you expect, since this is (...I think...) your first DSLR, double check that what you expect is what really should happen.
Also, take a test run of shots of exactly the same scene in "A" mode, full wide angle, at apertures ranging from wide open (eg, f/3.5) all the way to the smallest aperture that lens offers (ie, f/16, 22, or something in that range). One typical problem is the lubricant on the aperture blades failing and so the aperture sticks. If it's working properly, the exposure for all versions of the scene should be essentially identical. OTOH, if the aperture is sticking, it almost always sticks "open", so what will happen is that the photos at (supposedly) smaller apertures will be significantly brighter than those at wider apertures.
Good luck with it. IMHO, it's a can't-go-wrong camera purchase for you.
Tom