Bukkit

Is it possible to restrict certain items to certain groups (permissions?). If not this would be a great feature to add. (for example I could give the node dynamark.buy.1 or dynamark.buy.stone for players to buy ONLY stone, and no other item)

Would it work if there was an optional item configuration (say "group") where if 'group' existed in the config, then the player would have to be in a permission group of the same name?

the config node could look something like this

group:
  - member
  - admin

this would restrict the item to members and admins.

Yes this has the same effect in the end, just needs an extra reliance on your plugin. As long as there is some sort of /market reload command, it won't make a difference in the end, not sure if that exists yet or not.

It's a bit easier for you though, in my opinion, if you just check for the parent node dynamark.buy first, and if this is not found, then you can check for the specific node of the item they are looking to buy.

For example, if they want to buy stone:

Check for "dynamark.buy". If they have it, good, let them buy it (its a parent node, stone is allowed). If they don't have it, lets check if they have dynamark.buy.stone (or dynamark.buy.1). If they have that, let them buy it. If not, they are out of luck and not allowed to buy stone.