Some people want so that the currency has money sink so it cannot be converted back to coins and some people want platinum tokens like in OSRS, that you can transfer back.
Solution that works for both:
1. Create custom currency for the rare shop that cannot be transferred back to coins.
2. Add platinum tokens which work exactly like in OSRS. Useful for stakers & other richer players.
For the custom currency use reward token as example and rename it to "Rare token" or something.
http://oldschoolrunescape.wikia.com/wiki/Reward_token
