Grading System
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Note: almost all of these vintage paperbacks have some tanning of the paper from age. I don't count this as a defect unless the tanning is quite heavy.
Mint (M): The book is perfect in every way. I have never used this grade.
Near Mint (NM): The book looks like new, completely unused, with only 1 or possibly 2 tiny defects allowed - for example 1 tiny crease on a corner, or a couple of tiny scuffs on the spine. The adventure sheet might have been used in pencil and erased so that no marks are visible.
Very Good + (VG+): Along with NM, this grade is usually acceptable to serious collectors. The book looks almost as good as Near Mint, but there may be a couple more very small defects, or one defect that is more visible. For example a book with 1 deep, visible crease on the corner but no other defects would be VG+ - or a book that has several light creases to the spine but is perfect in all other ways.
Very Good (VG): Some collectors are fine with a VG grade, some aren't. A VG book will usually have a few visible but minor defects, or at least 1 serious one. For example a book that looks NM, but the adventure sheet has been used extensively in pen, would be VG. Or a book that has several creases on the cover on both sides, but is excellent in all other ways.
Good + (G+): A rare grade for books that are almost, but not quite, VG. They may look very good but have a couple of serious defects, or they may have many small defects (e.g. a library book where the cover has been well protected but is missing a page and has a few library stamps). Not much difference between this grade and Good.
Good (G): This book will still be perfectly usable and readable and may also look attractive, but has many minor defects and/or 2 or 3 major defects, such as a tear on the cover, permanent marks in the adventure sheet, or damp staining. Collectors will usually not be interested in this grade unless the book is quite rare.
Acceptable (A): This book is just a usable reading copy, and will usually have several major problems. The cover may be heavily worn, there may be tears and stains or damp damage, and the adventure sheet may have marks that can't be removed. However, the adventure should still be playable with no missing pages or unreadable passages.
Poor (P): I will almost never sell a book in this condition unless its extremely rare or has something else to recommend it such as an author signature. A book in Poor condition is essentially trashed. It may or may not still be usable depending on what you're willing to put up with.