I am just guessing, but I have been noticing a lot of photographers using CH as their default mode, and ending up with 5 frames of (what is most likely) near identical photos each time they wish to capture something. (I won't comment on such style of shooting)
If the seller is truly things this way, 3000 / 5 = 600 "real" shots per month, or 600 incidences worth taking photos of. That doesn't sound too bad.
Also, if the camera is confined to studio work, it should not be too bad either.
I note that my 1 year old Nikon D7000 has not crossed the 10k barrier yet, so the seller has used his / her D7000 more than 7 times over my usage.
My D7000 is 1 year old, heavily used for event photography has hit a shutter count of 50k already :O and I never motor drive. Exterior wise still looks good though haha
But I agree, by shutter count and age of camera, its a very good way of determining its usage.
Now... Since the D7100 is out, its time to upgrade to FF lo!