These condition codes are: New, Like New, Very Good, Good, Acceptable, Poor. In general, the only categories that you should consider buying are the first three, anything below Very Good is not worthwhile.
List of conditions
New: These books are exactly in the condition that one would purchase it from the producer.
Like New: Also called “Fine,” these books show some of the wearing that is typical of books that have been stored for some time. Some defects may include shelf wear or very faint dents on the cover.
Very Good: These books are worn, but still in excellent condition. This is the minimum level that I will buy books in with very few exceptions. There should not be structural problems with the book's spine, and no tears to the dust jacket.
Good: This is an average used book. The spine may be creased or worn, the bindings may be loose, there can be highlighting, or a crooked (“cocked”) spine
Acceptable: Also called “Fair,” all of the content pages are present but may lack the title pages or the end pages.
Poor: Also called a “Reader.” This book is still readable but the dust jacket may be falling apart.
In all cases, defects should be noted including writing inside the cover (e.g. the previous owner's name), bookplates, stamps, etc. There are a couple of qualifying circumstances that deserve special notation:
Ex-Library: If your book is a former library book, that needs to be marked. Books that have been rebound with “library bindings” are usually not worth your time.
Book Club: Book club editions (which are marked as such usually on the inside of the dust jacket or on the title page) must be noted as such. They are also largely not worth your time.