I wish the Shutterbug Classifieds were still worth looking at. If you can find an old print edition in a library, it's worth a look -- their rating guide was pretty much the basis for the condition rating that retailers still use. Something rated at 8 or higher, for instance, can't have any mechanical or optical defects, and the cosmetic difficulties are restricted to that inevitable greying that the "lizard skin" acquires over the years and some minor brassing (the black paint or plating has worn through at the corners and in areas where you'd expect a lot of handling). Just about every brick-and-mortar photo retailer with a used department I've been to over the past twenty years or so sticks pretty close to the old Shutterbug guidelines.

One thing you do need to pay close attention to with the Mamiyas (and with the Fuji 6x8), though, is the bellows. IIRC, you have a bit of time with view cameras, so you should be aware of what to look for (pinholes, light leaks, etc.). Make sure the body and back mate properly whenever you buy a back. And if you have the ability to check the shutters in the lenses for timing, that would be extra cool, but you can usually check the longer times pretty accurately and "earball" the rest (and like any mechanical shutter, expect them to be a little slow, but not outrageously so. You can always make up the difference in the aperture if you're within less than a half-stop.)