Say we collect Heroic Shield Eventide x20, Clown Nose Red x15, Tightroper's Dress Bee x12, and a bunch of other older event items in higher quantities.
Instead of selling them at the sell back store, we could get another option:
Trade them in for Older items that may have been released before we joined.
There could be set prices for them, for instance, One Observatory background costs 50 past items, 1 Pumpkin Pile from Halloween 2014 costs 75 past event items, etc. The older or more expensive an item was upon its release, the more items it would cost.
This would help people complete their collections, cut down the amount of extras they have in their inventory, and promote being active on future events so that users collect lots of extras for trading them in.