You can't stop making the currently tradable rares untradable. That is completely unfair to those that have put alot of their GP into getting them, not just GP hording merchants. The only feasible way I can think of controlling the prices of Rares is making a NPC Rares Trader that will buy your currently tradable rares from you fomr a set price to begin with, then depending on the Trader's current stock he would buy and sell for different prices. This way the Trader's prices would still be player controlled but it would remove the current price manipulation that occurs. I'm writing up a new thread to post about this later with more description of the idea :)