I'm really glad you are finding the exchange useful.
Bar selling will not be implemented, since bars are not considered raw materials and players can already sell ores instead. This also helps avoid cluttering the Bits and Bobs items menu. As for gems, selling them might be added in the future, but not for now. Gems are treated as rare materials, and I want to avoid a situation where players sell them off, only for a new item or mechanic to arrive that needs them, that would be really frustrating.
That said, it is normal for resources to eventually fill up over time. Even with the exchange system, there will come a point when materials like quartz (and others) will hit their limits too. In a way, that just means you love the game a lot, and I'm super thankful for that :)
