While grinding for treasure pieces it can be really annoying if you get the same piece more than once. For example I've got 3 iron fist warden pieces in storage collecting dust before I finally got the sordid piece. A guildy of mine had 9 of the same sulfur pieces before he finally got the last one. It would be great if two players could exchange two treasure pieces with each other so they can make use of their surplus pieces.

By only allowing treasure pieces to be exchanged with other treasure pieces it would also mean that you players can't buy a treasure. You still need to grind for them. Another limitation could be that you're only allowed to exchange pieces if they're of the same treasure. That way you can't keep grinding at lower AP/DP zones like pila ku and exchange for compass pieces which have a higher AP/DP requirement (though I think people would be overly optimistic if they think there will be people exchanging Elten pieces).