In my opinion it wont destablize the price but doing the oppsite. In the way it is now some class capes wont bought other will be alot. So thoose which are there often will drop in price as long as the offer lasts and may also after it. The class capes which actually wont be bought will be if needed really expensive. And there we come to the point the exchange npc will stabilze the price because when the balance of classes change for example soullinker or stargladiator becomes really playable their capes will be suddenly more worth then thana (maybe a little less because there are alternatives). So in the end I think an exchange npc just will result into stabilzation of price because it is no more combound to the class balance. Furthermore I think more people will decide to donate for it because its usefull for all classes but not just for the one you decide which makes people not donate for it. And also if you say you was going to make it one item its just the same with the exchange npc. If you decide now to do it wont harm anyone because noone has it yet. And you cant compare it to belts or valk weapons because they are almost doing the simular thing to every class in a way and you actually can use more then 1 type of belts on a class and about the weapon it is clear that you cant exchange them because weapons always should be class specific and you can donate them always. So I agree to the sugesstion.