I've been referring chapter 9 of Humphery Sheil's study guide and have noticed that not all the candidate entities have been annotated as @Entity in the class diagram (Bidder, Seller, User). Any thoughts?
I would urge you to not focus too much on the provided class diagram. It is only meant to provide an idea about the level of detail expected by the part 2 assessors. Ultimately, it is the job of the architect to justify his architecture. If you think that a specific class is best implemented as an entity bean in your architecture, you should depict it as such regardless of how it is done in chapter 9 of the book.