Go by Pinsides rating. Here: http://pinside.com/pinball/forum/topic/print-off-for-rating-pinball-machine

I have no formula or numbers, I don't believe numbers ratings help sell a machine, if a pin has a flaw then it should be stated and the buyer made aware.

I believe it's right to just give a very detailed description included all defects and marks etc outside of the normal(minor) age related blemishes, so if there is wear mention it, chipped, cracked plastics, fade and so on. I think it's good to tell the buyer every bit of work and the kind of maintenance you've done, also to include loads of pics from every angle of every part inside and out and close up shots, that way the buyer will have no surprises and no excuse to back out of the deal. Same as if you ask a seller questions if you don't get the right response then what could they be hiding.

If you deal open and honestly with people, you're building relationships with other pinheads and your name and dealings will be trusted and passed on to others, If you have a great buying experience with some one your likely to work with them again.

i am with you, I do not use anything but whole numbers if I even use then at all. When somebody asks condition I tell them my feelings and make sure they know it is my opinion and that they should really look at it in person.