A situation may arise where you have spare copies of one element, but don't have another element at all. For example, you have a lot of Troll Bones, but have trouble getting a Troll Tooth. You can find someone who has spare Troll Teeth and exchange some of your Troll Bones for their Troll Teeth. Most collections can be exchanged, but there are several collections that can't be exchanged. Such collections give you rare and powerful items, for example, Gladiator Spear and Wyrm collections. If there's no “Exchange” button in the collection block then you can't exchange elements of this collection.
To exchange an element of a collection, choose a collection you want elements for and click its “Exchange button”. A window will appear where you can choose elements you want to give and elements you want to receive.
You can exchange items of one collection for items of different collections or alchemy ingredients.