My suggestion is to change this item a little bit.
The Yadrat warehouse key opens a bank slot, that can be accesable between characters on the same account.
My suggestion is too allow all possible items to be put inside of it.
-Dogi's with effects.
-Items from CC.
-Items from BIDS.
-Items from shenron.
-dogi balls/brown boxes
If you do this, people can use these items on their OWN characters only, and still cannot sell or trade it away.
Advantages
-People can make characters that are specialized for PvM, and trade rewards to their other characters
-If people make more PvM characters, population in this game, will have a much better time finding and creating teams for PvM.
-If class rebalance happens, and people dont like it anymore, they can swap items they worked so hard for, to a new character of their chosing. (less people quit/chargeback)