I agree with some points you made, and disagree with others, Dielan.
While I think having more DHX shops would be beneficial, it would be putting money into the wrong people's pockets. What we would see is large groups of people camping the shops and waiting for them to change, or have people with large reserves of items just farming those shops. It would have to be highly regulated.
One way to combat this is by essentially creating a stock market for the items. The more items that are in the chest, the less coin you can sell it for. If the chest is empty, you will be paid more.
There was a plugin for something similar to this a whole ago, although I have long forgotten the name.
As for item taxes... It doesn't work as planned, IMO. When LO got kingdom status, we farmed enough materials to outlast the life of this map. We didn't buy any materials, we used professions from within our kingdom to the materials. In no way, shape or form did it contribute towards the economy.
That's the whole point of a kingdom, though. A community that works towards a common goal. Because of this everyone is willing to help, no coin required.
The new DHX rare good shop is a step in the right direction to take coin out of the game. All that is needed now is for those items to be required for certain things such as upgrades.
(Sorry for spelling an if this doesn't make sense. Typing it fast on my phone at work)