To assist in describing some one or actually collaboratively this group should set up standards for descriptions and possibly grading of a machine for objective assessment of a machine when selling or trading pinball machines. For example, criteria for giving a grade for the playfield...like glossy colors, no scratches, no insert damage or lifting and pics of these things could be included so the newbie could understand the important features of how to grade. To be even more thorough, pics of almost every machine in different states to demonstrate these differences could be shown. Maybe this could even be part of the IPDB
For cabinets, pics could be posted to show and objectively evaluate cabinet fading, nicks or scratches etc.
Many categories could be evaluated to give a pinball an overall score.
Then when a pinball is listed an objective score could be used like playfield B+ by pinside universal pinball score. Even better would be a check list where each category starts out with a score of 100 and for every type of deterioration or damage points are subtracted per and objective score

Just a thought...

I think to avoid the initial arguments a small committee or a well intentioned seasoned collector /seller/buyer should set up the criteria....from there suggestions could be considered but not necessarily taken. If it is built fairly without too much bias towards buyer or seller than I think it would eventually stand up. I am sure as you stated initially it would be rather controversial. It would have to be someone's project for a while I would expect.

if someone adopts this project who has enough passion and time, I believe the guide could give the person some pinball fame by allowing that person's name to be used in the name of the guide...much like "kelly" blue book, Clay,s Pinball repair guide, Scoville scale for food heat, etc... It could be someones lifelong recognition in the pinball community....recognition that newbies and old timers would know for decades