i've only purchased one lens from keh, it was an olympus 24mm 2.8 prime, that was rated as ex.
the lens looked good on initial inspection, but after taking a closer look with a flashlight, i noticed that the glass wasn't really all that clean.
after i finished cleaning it with ror and kim wipes, i found light haze under the rear element, covering about 20% of the glass.
then i noticed that the blades were sluggish, but only when opening up to f2.8... they were dragging pretty severely only at that spot.
they took it back, and i ended up paying an extra $26 or so more for an ex+ version of the glass, that so far looks much better.
ever hear the old adage that no one ever sells a good car? i think that it also applies to camera lenses