First of all a big sorry for the inactivity! Here’s the plugin from Preview #5.
It allows you to set purchase and sale prices way more freely and dynamically than the default Maker. You can have “tags” on your items (e.g. “wooden”, “sword”, “potion”, …) to change prices for a whole category of items at once. Also there are notetags to change prices as well as plugin commands. You can easily reset made changes with plugin commands as well. Thus it’s easy to change prices on the fly.