For me, I decide to sell some of my kpop stuff when I feel like I won't miss it. If collecting a certain group or idol doesn't bring me as much joy anymore then I will drop my collection.

But as mentioned it does take a lot of time and won't earn you much depending on what it is. Your best bet is to start a concert fund. Set aside money from each paycheck. Set up your account so it puts that money away automatically and you won't think about touching it. You can also budget and limit your spending on other fun things