Zic, your cards/luna aren't in danger when you need to bank them. Here's how it works:
1. You initiate the "trade" with any of your allies.
2. Your ally has two choices:
2a. Press "Cancel trade," successfully banking your items in your Inbox.
2b. They try to offer you something in return for your items. If they do this, the ball ALWAYS goes back to your court (so long as YOU initiated the trade), and you have the choice to either Confirm the trade or to reject the trade.
Again, your stuff isn't at risk so long as you initiate the trade, and usually people are more than happy to hit cancel for you.