As a general rule, for a collectible modern children's book, the least acceptable condition for books I buy or sell, is Very Good in a Good dust jacket. Of course there are always books that are exceptions to the rule, see even as x-libris .
This is not so much elitist as practical, by the time all the faults in a book and dj are described, (bumped corners, foxing, dj tears, names or scribbles by previous owners etc.) only the most hard to find of titles are going to be attractive to the book buyer, especially for an internet sale where the buyer can't examine the book until after purchase.
My book conditions in descending order are:
AS NEW: I very rarely use this, I've had books new from the publisher that I won't call As New. ( The term Mint is for coins and chocolates, not books).
Fine: Is in unread or very lightly read condition. Dj is unmarred and not clipped.
Near Fine: I know a lot of dealers don't use this term, but dang it, a book with lightly bumped corners or a small crimp to the boards is not good enough to be Fine but too good to be Very Good. For a dust jacket to be Near Fine it may be slightly shelf rubbed, be price clipped or have a very small closed tear. (NOT all at the same time).
Very Good: May have previous owners inscription, light foxing (yellowing) bumped corners etc. But all major detractions should be listed in the description. Dj may be sun faded or lightly to moderately soiled. I've been known to use a plus and minus sign, IE Very Good+ for a book that is in better condition than Very Good, but not quite near fine. This also is not common practice for all booksellers.
Good: For collectible modern children's books Good is not good. Examples of books that fall into this category are, X-libris, (library discard), books that are foxed, have shaken bindings or are heavily marked and soiled; the book is intact and readable but it better be pretty darned desirable to make it worth having in this condition. A Good condition dust jacket may have tears or chips missing, be heavily soiled or sun faded. As with the Very Good category the major defects should be described.
I won't buy or sell books in Poor or Fair condition. I don't have the room to store them or the sales technique to describe them well enough that someone would want to buy them.