If you're going to sell used books either online or to a bookstore, there are several things you need to look for in the condition of the book.
What is the overall condition of the book, as in, what's the first impression you receive when you look at it? From here, you can focus in on how to rate the condition of your book.
First, look at the cover. If it was purchased with a dust jacket, make sure it is still on the book, and the condition of the jacket. Determine if the edges are frayed, either on the book cover itself or on the jacket. Take note of any creases on the cover - by the corners, in the middle, near the spine. Also look at the corners of the book. The corners of paperback books, especially, have a tendency to begin to turn upwards with use. Slight indents on the cover can also be categorized as normal wear, or slight wear. If there is only slight fraying on the edges of the cover, this may be due to shelf wear; the wear a book receives from being placed on and removed from a shelf. This is normal wear.
Second, look at the back cover of the book. Review it for the same symptoms as the front cover.
The spine of the book is very important, as it will keep the pages intact in the book. Slight creasing of the spine is normal with some used books. Make sure to take note of this condition as well.
Most importantly, review the inside pages. Make sure to document any kind of markings, like highlighting of text, folded page corners or dog-ears, and missing pages. Optimally, all the pages should still be bound together along the spine.
Once you've reviewed the necessary aspects of the book, you're ready to accurately list your book online, or take it to a used book dealer. Book dealers are never the same; some will always be more picky than others about the conditions of books. Make sure you find a dealer that is happy to accept your book in it's current condition!