I learned grading through Overstreet some 30 years ago & still keep a guide handy to this day. People like to grade for themselves though, so all books over $10 will have front & back scans, some in the $5-10 range will too, cheapies just have generic thumbnails.
Some grading notes:
Assume all books are complete & attached unless stated otherwise OR the flaw in question is normal for the grade.
I allow detached covers up to GD, partly detached up to GVG. Centerfolds detached up to GD+, partly detached up to VG+.
Missing stamps: This flaw will always be mentioned.
Subscription Folds: Any full length color-breaking fold usually means GVG but sometimes VG if the rest of the book is in nice shape.
Water damage: Obscure, flat dime-sized spots allowed up to FN+, trace amounts allowed into higher grades but will be noted. Undisclosed w/d is a pet peeve of mine.
Books in GD can be read normally, books in FR with difficulty. Books typically drop below GD if they're incomplete, falling apart, badly warped, or have something gross like mold.