So someone could have a lot of store items, and they don't really use a lot of it. For example someone got a pet airplane, then later they got an even better pet like a pet phoenix, then they would only use their phoenix and their paper airplane would sit there. So this way the paper airplane pet wont just sit there, and they could sell it, or trade it for a collectable, or trade a lot of their unwanted store items for an even better store item. For the person trading for the item in store, the trade will be way easier to do, and they won't have to worry about getting scammed. So of course you can't trade the stuff like prefixes, but just items would be way more easier.
