Our description of the physical condition of an item begins with a description of the overall condition of the item followed by any flaws which would not normally be associated with that overall condition. Whatever conventions are used, it is important that the buyer understands what the seller means. If you have questions, ask before you buy. Reputable dealers won't mind explaining what they mean by the terms they use or elaborating a description to a potential customer. Pricing books is not easy or trivial.
Dealers rely mostly on experience.
Do they have a number of customers looking for that book, or have other dealers been looking for that book. How much does the condition of this copy vary from the other copies the dealer has seen. Dealers gain experience by actually pricing books and trying to sell them. Remember, you must compare condition as well as price. Price guides can also help establish the value of a book.