You can choose to rent your cards or play them. Obviously you will do what is best for you at each time you want to make that decision.

Switching between both those has a cost. If you have 100 cards you are playing, then its 100 DEC each time you want to go from playing to renting. That's about 7c ... I don't know how much you get from renting but if its not at least 7c then its probably best not to rent.

Obviously there's a cost to the game every time everyone makes a transaction. In the past they didn't charge anything and it was abused. People found loopholes and listed millions of cards many times each day. In other words humans used tools to find ways to get every penny out they could and didn't care what happened to everyone else. This will make things more fair for everyone, including you. It will make using the resources of the team more rationale and thus people will only do things that are "worthwhile". Contrast that to the fact that people exploited things when everything was free to do.