I think it should be linked to the main inventory.
For example, you could have a coin collection.
Coins could be found in the stores, or dropped by pets. Or even both.
You have a new inventory, the collection inventory like intorain said, but you also have a new 'use' function. So that they can put it in the new inventory. But if the item is already in there, it doesn't place it in the collection inventory. Then, when they have each item in the collection, you could make it that they have to 'claim' the prize. Which checks that they have all the items needed in the collection, which then gives them a prize if they do. It could also lock the collection, so that it's finished, or even reset it.
Because you don't want users to be able to finish the collection, claim the prize, and then sell the collection. Or even claim the prize multiple times without collecting the collection again.
Hope I make sense @.@
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