Suggested by lukeeexd:

With lots of shops on the server, it gets hard to find what items you want. Idea: Browse by item.
If you open item browse it shows every item being bought, sold or traded for how much and by who.

My remarks:

As mentioned here, localized/user-friendly item names would need to be avoided as much as possible.

One way I could see this implemented could be, like said, as gui that display all traded items, sorted by item type. A top level category could be to differentiate between 'sold' and 'bought' items.

To make it easier to browse, the items could be divided into categories. These could be a fixed limited set of general and potentially large categories supported by Bukkit API ('is block', 'is item', 'is tool', 'is enchantment book'). It might also be possible to extract the item categories used by the creative menu).

The alternative would be to allow specifying the categories via a config option, requiring the admin to list all items that belong to a certain category. The config option would only allow grouping of items by item type (not by item attributes such as enchantment type, name, lore, etc.) in order to keep it simple to implement and maintain (some of these options could maybe be added later). And the option would not be used by default, since I want to avoid maintaining such lists myself (or the default simply implements the general categories, in which case more advanced filters than by item type would be required from the beginning).
Any item without a specified category would go into a default 'unknown/various' category.