I think it's what the buyer is willing to tolerate, much like anything else. Many have a particular book they are willing to buy. Especially for a high ticket item as AF15. When I look at a SA book specifically, it has ALWAYS been this:
1. No ink/stains/smudges/Initials/Names on cover (I do love those old school splash panel artist sigs though!) 2. No MC 3. No mis-wrap. No date stamp (unless on the BC) 4. No front or back staples (depending on book).
Of course, there are other things to consider based on the particular SA book you're looking for (X1, TOS39, etc.) and I won't even get into that here. Just know, these books are only worth what someone is willing to pay for them.....albeit, for the adjusted price in said grade.