So I was going through my bag and I have just under 2700 music boxes. Now I know this event was back in April I believe. I was wondering if this event will come back around or should I sell this for over 33 million?
So far to date the developers have not reused event items nor have they repeated an event that will required items previously used.
Until they advise us otherwise you have the choice of selling them if you need the gold for another event like the Blacksmith event or holding on to them until you need the gold.
As your bag has unlimited storage they is no reason why they can not stay sitting in there until needed. As with the recent anniversary event they made a special event in which you can use your leftover Koi to exchange for items or sell them from the bag. This appears to be a newish event option they have that comes out at the same time they make the old items sellable.