Right now all they have is another type of store. Collect a bunch of junk to clutter up our inventories so that we can buy items that aren't that good to start with from a store, buy enough apparently and it unlocks the next tier.
Sadly, by the time you collect enough stuff to buy an item from the store you are pretty close to the next tier in level already so forget about collecting stuff to unlock that store.
Come on already if anything it should be buy something from the store and move up to the next tier. It's not like players are going to exploit getting access to the next stores items before they get the level that can actually use the items.