The progression concerns you brought up were not something I factored into my post, and I agree with your points on that. This was why I couched my suggestion under the (almost) anything idea. I think you can be selective in what you make tradeable and still create a robust economy.
Aside from progression, I think we can all agree that there is a huge disparity between the number of desirable, tradeable items and items that are no trade. Money in any economy is worthless if there is little or nothing to buy. For example, after WWI, the Germans burned Deutschemarks because they were more valuable as a heating source than as a means to acquire food and other necessities. Platinum itself has no intrinsic value unless it represents a means of barter that is accepted by the player base, and right now we have hyper inflation due to a low supply of higher-end tradeable items in the market and a glut of platinum sitting in people's banks.
I don't know what an appropriate balance would be as far as which items should be marked tradeable, or whether that is even feasible from a limiting progression standpoint. What I do know is that without more tradeable items in the economy, platinum will continue to be perceived as worthless.
Kind Regards,
Josh Wilhelmi a.k.a. Cunning