I know what you're thinking.
"It's too easy to make tons of accounts and do the first few quests for easy coins"
But it's also easy to make the quest coins nontransferable. So main problem solved.
Allowing card exchange and gold transfer would add an economy to the game that would make it alot more interesting to win certain cards. Especially when you don't need them. And that wouldn't be the only perk. An economy in any game, makes for a much more interesting game overall.
For example. Upgrading a card costs 1500, but selling it back you get between 1150 and 1300 gold. You could sell your upped cards for 1400 to other users. If they're rare cards, for even more. Card trades, etc.
I'm surprised this hasn't be done yet, to be honest.
Hopefully this is something that is positively received by the community as I'd much like to see it happen.