I know this idea has been suggested before, but I have a feeling necroing 6 month old threads will get them locked.
2 main reasons why they should be tradeable.
These are an integral part of both Sorcerers and Necromancers. Who have to be around for field bosses and even then generally only have a 10% chance of getting the card as a drop.
Cards can only be upgraded by using other cards. At the very least gems should be able to be used to upgrade boss cards as well as we end up sacrificing an already scarce resource of ours.
I see no reason why cards are not tradeable/marketable.
tl:dr trade/sell cards because sorcerers/necromancers get unfairly hit by it