This have already been brought up numerous times over the past few months now, and have already been explained on every occasion as well that it will never be implemented due to the possibilities of players abusing such a system.
I think you meant a trading system between players. What I suggest is kinda like the shops, but you use the items that you can't use anymore since you've passed the requirements to use them as currency instead.
(Ex. Common Thread)
Although as much as I want a system like that, you're right as it would be exploited too much and be unfair.
Both trading between players and trading with shops have been suggested and brought up, and for both of them Tobei have came forth to make the statement that it will not be implemented.