But I've already worked so hard for all my upped cards
I feel that if rares were ever tradable, it could still be abused a lot because it's not very hard to give the rares to someone else so that they have a set of 6, then let that person farm for even more of the rares. Same with shards.
I might like the idea of trading upped non-rares for other upped non-rares.
And no buying cards from others for
. It has to be 1 upped card for another.
buying cards for
would just fill up the chats will spammers trying to get a good deal.
Chat crowding is definitely something to consider.
Edit: I just thought of another problem/comment. Most people tend to upgrade the same cards, like Supernova. So having 7 SNs from FGs would usually still make the player just sell the 7th card. So it wouldn't be very good for the completionist who is only trying to get the upped cards he can't already win. Nobody is really going to have a 7th say... antlion.. if i wanted one. ('cept i have 6 already, and they're awesome)