I still don't get the stars. You are going to lose stars 31 days after your last purchase and on winning a tournament, you will lose it after 7 days without winnig, is that right?
Purchase: display whatever number of stars you got for 31 days (they all disappear after that, but you'll get them back
when you purchase again)
Win a tournament: get 1 more star, last for 7 days.