I have bought and mastered all of the equipment that is available for sale in the item shop, but there are a whole bunch of items that are greyed out. I have tried equipping lots of weapon/item drop rate increase gems, but invariably this yields low quality equipment that I already own. I have had a small amount of success with prize wheels, but they too primarily yield gems or equipment I already have. Am I missing something?
After further experimentation, Rare Prize Wheels seem to be the way forward. You occasionally get stiffed with a potion or a gem, but the majority of the time you seem to get a piece of a equipment that you do not already have. When you reach the point where there are significantly fewer items that you do not own compared to items that you do own, you will start to get duplicates, even from the Prize Wheels.
I am posting an answer since I can't comment.
I think there are maps which unlock additional weapons too. You can buy these maps at the item store, which show you a location marked by a cross -> X <- You simply tap on the location, in the world of IB, and you will get the weapon or whatever the map points to.
There are 5 maps in IB2 I think.
Hope this helps!