I think the ultimate problem is that this hobby has no standardized grading system...which is odd, because almost every other hobby/collectible item DOES have some sort of grading system.
I propose that we on pinside (as the pre-eminent pinball community) attempt to make one.
Here is my proposal for basic and simple standardized grading system:
10 - New or Mint pinball. new in box, old in box, or restored pinball (full original status which includes all new parts and pieces or ones that have been cleaned to the point of appearing like new)
9 - Like new or near mint pinball. Single owner game with low plays or mostly restored game (Like new game with very minor flaws)
8 - Excellent used condition pinball. Used but 100% working and clean. Will show some wear, but it is very light.
7 - Fine used condition pinball. Used but mostly working and clean. It may need small tweaks to get to 100% working and/or clean. Light wear.
6 - Very good used condition - Used and mostly working. Will need some items to get it to 100%. May need a light cleaning. Noticeable wear, but still worthy.
5 - Good used condition - Used and mostly working, but showing moderate wear. Might be missing a couple of items or need very minor repairs that don't effect gameplay. Needs cleaning.
4 - Players condition used game - Used and working to the point of being playable, but needs obvious fixes. Might be missing a few items or in need of repairs, but it is still functional. Needs cleaning, repairs, and touch ups. Moderate to heavy wear.
3 - Non-working game - Used and not working. It needs immediate attention to repairs to be functional. Will need cleaning and touch ups. Moderate to heavy wear.
2 - Project game - Used and not working. In need of multiple repairs and/or replacement of missing parts. Likely has several broken/cracked parts. Needs a deep cleaning and touch ups. Heavy wear in areas.
1 - Complete project game - Used and not working. Missing major parts and in need of major repairs. Many broken and cracked pieces. In need of major cleaning and touch ups. Heavy wear.