Bukkit

Make it so you can setup all items that you want to buy/sell in the market, including custom item name strings, instead of only allowing ones that are statically codded into the plugin. This would make the plugin much much smaller as you wouldn't really need any of the item code coded into the plugin. This would make the plugin to some degree update proof, you wouldn't have to modify the plugin to add new items. Still any items not specified in the config file would not be recognized. This would be easier to do once sqlite/mysql is supported as you can store the item prices separately from the item config data.

The format of the items in the config file would have to change to really handle this very well, you have to have the item records go by item id's instead of item names.

Something like this for the item record in the config file, if item prices are separate from the item record. For "initialprice", if it's not specified, you could make it use a global value set in a config file. And allow initialprice to be used in the data: items, and override the items master value if it is set.
'278':
  initialprice: 100.0
  isdurable: true
  blacklisted: false
  names:
    - diamondpickaxe

or

'17':
  initialprice: 2.0
  isdurable: false
  blacklisted: false
  data:
    0:
      names:
        - log
        - oaklog
    1:
      initialprice: 3.0
      names:
        - sprucelog
etc...

And potentially a command to add an item in game. Although I personally prefer to go through the config files for something like that.