As someone who both rents cards to use and rents cards out to others, I completely understand the issue - but its a two way street. I've often rented out cards before only to have them cancelled the day after the new season starts and the people collect their rewards. While having the flexibility to cancel on any day has potential value to both those who rent and those who are renting, in many cases people seek more certainty. There are many potential fixes, but the simplest might be to simply split rentals up into two catagories - daily and weekly. Daily rentals would continue to work in the same fashion where either party could cancel at any time, while weekly rentals would have both parties locked in for the full 7 days at the agreed upon price.