Shadowverse: Rise of Bahamut

If you only implement an auction system instead of trading, and introduce a collateral system, it won't break the game. You won't be able to funnel money/cards to your main account without the fear that someone will outbid you on an item while you're logged out. And if you also tie access to a specific rank, it will make alt account manipulation even more inconvenient. That's just my opinion.