I have been reading through this since the thread's inception.
I personally subscribe to the old model of scans with the older grading system for raws. G, VG, Fine, VF, NM. If I feel a book is between a grade I will say VF+ or VF-NM.
I cannot stand behind a point system as I have personally seen books come back higher on straight resubmits by a full point.
Recently acted as a third party in the sale of a book. The seller estimated the book at a 5.0. Both the buyer and myself estimated the book at a 6.0 with a 6.5 on a good day of drinking. The buyer has a history of some spot on grading.
Book came back a 7.5.
I feel it's safer all around to not guarantee an exact grade on eBay. On the boards, we act under the assumption of familiarity with the CGC grading process so it is a different story. Tuesday's 8.5 could be Wednesday's 9.0 and Thursday's 8.0.