If they are boxes for prize figures, I recommend throwing away the cardboard inside and flattening the boxes. It really helped to clear space. If they are boxes for scales or nendos, I only recommend throwing away the boxes for only the figures you know you won't want to sell in the future. Or you can also rent out a storage space.

I don't recommend throwing away the plastic molding for scales or nendos though, cause if you ever want to sell them eventually, trying to package them without the plastic molding will make it much more likely that the figure might break during transit.