Rather than give us trading of rarities, I would personally prefer a group crafting/heating station. it could work in two ways - you group up with players at the station and heat/craft items together (my preferred method), or players can donate excess rarities and materials into a station which then anyone in Spiral Knights can use to level up or craft their gear.

Costs at these stations would be somewhere around x1.5 to 1.66x the amount of regular crafting costs, meaning if you want the cheapest items you're crafting on your own. Group crafting would be for players such as myself who have too many materials, fire crystals, and orbs of alchemy, and would be more than happy to help friends and strangers alike heat or craft their gear.

This essentially creates a greater sink for these items without having a direct influence over their market. Since group crafting costs more overall you'll really only find players with too much stuff willing to craft things, or additionally, if you're playing with a group of friends, you can help your friend get to the next star level with the extra orbs you've gotten.

Selling the items isn't really needed, however all rarities are literally only in the game to be used by the player who finds them. By making the player able to use these items on other players, it keeps to this logic instead of just becoming "material 2.0".