I moved this to Classifieds from where it was originally posted and thought about whether it needed prices at the time, but decided to let it ride as is.
In general I think it better and preferable to list prices, but currently the rules are phrased that it is a guideline that sellers post clear prices.
It is one of those cases where the rules are a little loose, but if a lot of people started posting items with Price on Application the rules would be nailed down to read "prices shall be clearly stated by the seller", so as to avoid any issues with price gouging.